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Leanne Cartwright, seen here with her children Lewis, Alistair, and Jamie Lee, attend the London Marathon in spite of security fears.

LONDON – Defying fears instilled by Monday's deadly Boston bombings, boisterous crowds turned out Sunday to cheer on an estimated 36,000 runners in the London Marathon, many of whom wore black ribbons to honor the dead and wounded.

Hundreds of extra police offers were deployed to beef up security for the first major road race since three people were killed and scores were wounded near the finish line of the Boston marathon Monday.

Some 650,000 spectators lined the streets of London as Prince Harry, grandson of Queens Elizabeth, waited at the finish line near Buckingham palace to hand out medals to winners.

"People have been saying they haven't seen crowds like this for eight years around the route, which is remarkable to see,” he told the BBC.  “The way that Boston has dealt with it has been absolutely remarkable. It's never going to get anyone down here."

Local officials said that heightened security measures were in place to reassure the public and not in response to any specific threat. London's Metropolitan Police Service said it increased the number of officers on the streets by 40 percent. Sniffer dogs were out in force and trash bins had been removed from the course.

"The enhancement to policing, which will see several hundred additional officers on the streets, is intended to provide visible reassurance to the participants and spectators alike," the police said on their website.

After the twin blasts in Boston, some Britons, like Leanne and Richard Cartwright, decided not to attend Sunday's marathon in London. But the two avid runners changed their minds when they witnessed the groundswell of support for the victims of the U.S. attacks. The couple from Shrewsbury, a town in the west of England, brought their three children: Lewis, nine, Alistair, 13, and Jamie Lee, 14.

"The day it happened we decided not to come, decided not to risk it," said Leanne, 33, as she watched the racers in London. "The runners have to be here so we decided we should come and support them and show our respect to people of Boston as well."

Before setting off, runners wearing black ribbons marked 30 seconds of silence to honor the victims of the Boston bombings. The pack of race competitors bowed their heads and stood silently at the starting line, then clapped and cheered when a whistle marked the end of the tribute.

Related story: London marathon kicks off with a moment of silence

Moments later, the elite runners set off at the head of the pack. Thousands more soon fell in behind them, chasing personal goals or raising money for charity, many running in fancy dress. Small children reached out to high-five runners as they went past, spectators cheered elite and anonymous runners with equal enthusiasm, and a brass band near the starting line jokingly complained the boisterous crowds were drowning out their music.

"It was incredible, the amount of support, people coming out from everywhere, just cheering the whole way. Unbelievable," said a breathless Mo Farah, Britain's 5,000 and 10,000-meter Olympic champion, after running the first half of the course.

Farah ran half the route to prepare for competing next year.

Despite somber reminders of the deadly attacks on one of Boston's most iconic sporting events, the mood at the London race was overwhelmingly one of celebration. 

Some runners wore vests emblazoned with the city's name. One spectator held up a placard along the race route that read "Come on London, do it for Boston!"

Chris Jackson / Getty Images

Tatyana McFadden, an elite wheelchair athlete who won the Boston women's wheelchair race just before the two bombs exploded, said she hoped that she would inspire those injured in Boston by competing in London.

"The best ever! A lot of emotion because of the Boston marathon," said Nathan Comer, 38, catching his breath just after finishing his third London Marathon.

"The silence before the marathon was beautiful ... it just felt as if everyone was together," he said.

London's 26-mile course starts in leafy Greenwich, crosses Tower Bridge, snakes through the Canary Wharf business district before heading to Big Ben and finally Buckingham Palace.

Tatyana McFadden, an elite wheelchair athlete who won the Boston women's wheelchair race just before the two bombs exploded, said she hoped that she would inspire those injured in Boston by competing in London.

"That’s what we need to give back to the people in Boston," she said. "Running for London, I'm going to be dedicating the race to Boston. It's about showing that we're not going to let this event win. We will continue and we will fight on."

Reuters contributed to this report.

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Ammar Awad / Reuters

Racers reach the finish line of the first Palestinian marathon in the West Bank town of Bethlehem on Sunday.

BETHLEHEM, West Bank — Jesus' traditional birthplace has long been linked to tensions between Israel and the Palestinians. On Sunday, around 650 athletes took a step towards transforming Bethlehem's modern image by running in the first official Palestinian marathon.

“It’s a strange site to see people run in the West Bank,” said runner Dina Khuri, 21, from Beit Sahour.  “Usually when people run here it has to do with violence, but this time it’s for fun."

Courtesy Dina Khouri

Dina Khouri, left, took part in an historic marathon in the West Bank on Sunday.

Security was tight in the wake of the Boston attacks that killed three and injured more than 170 on Monday. Before the event, organizers said they were "deeply saddened by the news from Boston."

For George Zeidan from Jerusalem, the marathon was not only about fitness.

"Sports (are) my inspiration and my way of identifying myself," he wrote in a blog posted on the marathon website before the race. "I am running for the freedom of Palestine and my people.  I am running to inform everybody that we Palestinians are just like everyone else, we run, dance, sing, play, jump, and have fun, not only that but we are also good at it."

The race started and finished at the Church of the Nativity, thought to be the oldest continuously operating Christian church, and took runners in four loops around Bethlehem. Competitors ran through the Al Ayda and Dheisheh camps, which house 17,700 Palestinian refugees.

The participants from 28 countries passed the barrier separating the West Bank and Israel, which Israelis call the separation wall and Palestinians refer to as the apartheid wall.  The turn-around point was an Israeli Army checkpoint on the road leading to Jerusalem.   

Palestinians Abed El Naser Awajneh and Christine Gebler won the men and women's races, respectively, according to organizers.  

Organizer Signe Fischer said that the main message behind holding the race was to emphasize the importance of freedom of movement. 

“The situation here is of stalemate and this marathon is a positive message for people to do something to change things," she said.

While the United Nations organized a marathon in the Hamas-controlled enclave of Gaza in May, 2011, Sunday's race was the first Palestinian marathon. The West Bank-based Palestinian Authority gained non-member state status for Palestine at the United Nations General Assembly in November, 2012.  

Alongside growing tensions between Israel and the Palestinians, who have been struggling for years to establish an independent state, have come growing restrictions on their ability to travel into Israel and Gaza. 

Earlier in the week Israelis refused to allow some 20 runners from Gaza to travel to the race. Gaza residents can only go to the West Bank for "exceptional humanitarian reasons with an emphasis on urgent medical cases," Israeli officials were quoted with saying. 

This was likely a big blow to Gaza resident Nader Al-Masri who represented Palestine in the 5,000-meter race at the 2008 Olympics in Beijing.

Reuters and The Associated Press contributed to this report.

Related: 

London marathon kicks off with a moment of silence

'We will fight on': London Marathon competitors, spectators defy security fears

Andy Rain / EPA

A police dog handler checks a vehicle for explosives along the Mall in London on Saturday. The Mall will stage the finish line of the London Marathon on Sunday.

LONDON – The London Marathon: 36,000 runners, 650,000 spectators, and now 40 percent more police officers. The twin blasts that killed three and wounded 176 at the Boston Marathon Monday have prompted authorities in the U.K. to boost security at Sunday’s event, with about 700 additional security officers expected along the 26.2-mile long route. 

There are already over 33,000 police in London, which is no stranger to terrorist attacks.

"There are no gaps because obviously we police this every year with the London marathon and our other agencies - British Transport Police, London Police,” the head of the Metropolitan Police's London marathon operation Julia Pendry told NBC News’ U.K. partner ITV News.

"What we have done since Boston is we've reviewed our plans, we've reviewed our contingencies and I have put on increased numbers of officers in high visibility reassurance patrols,” she added.

The increase was meant to “make sure that when people step into the capital on Sunday they feel hopefully reassured and safe,” she said.

While U.K. officials have years of experience coping with security challenges – London has spent decades under the threat of bombs, first from Irish Republicans and more recently jihadists –a marathon is by its nature a potential security headache.

The race route is set to take runners past important and iconic sites – including Tower Bridge, the Houses of Parliament and Buckingham Palace – as well as through Canary Wharf, the riverside financial district targeted by the Irish militants in the 1990s. 

In a statement released on Saturday, London Marathon chief executive Nick Bitel also acknowledged “additional security issues ahead of Boston, but that seems be bedding down and the message of reassurance to runners very well received.”

Related: Runners, fans determined not to shy away from London Marathon

Related: London beefs up marathon security

Marathon organizers and security officials would not go into great detail about security preparations, but they would likely have been watching the dramatic events unfold in Boston.

Slideshow: Search for suspects in Boston Marathon bombings

Jared Wickerham / Getty Images

Cheers filled the streets after a Boston Marathon bombing suspect was captured alive but wounded Friday night — following a daylong manhunt that shut down the city.

Launch slideshow

On Friday night, Boston police captured bombing suspect Dzhokhar Tsarnaev, 19. The arrest capped a manhunt that had the city of Boston and its suburbs locked down after the execution of a college campus patrol officer, a carjacking and the death of Tsarnaev's 26-year-old brother, Tamerlan, during a 200-bullet confrontation with cops.

British officials were keen to point out that there had been no change to the threat level and nothing to link the London marathon to the two bombings in Boston.

“There is no change in the threat in London,” Pendry said.

Despite any preparations, it would be impossible to make the marathon completely immune to attack, according to security expert Margaret Gilmore.

“You can’t ever guarantee safety,” said Gilmore, a senior research fellow specializing in homeland security at Royal United Services Institute, the U.K.'s top defense and security think tank.

Organizers were also planning to commemorate those killed and injured in Boston by holding a 30-second silence before each of the race’s three starts and handing out black ribbons to every runner. They also announced that they would donate around $3 to the Boston First fund for every finisher.

The Associated Press contributed to this report.

Related:

What's next: The interrogation of the Boston bombing suspect

How thermal imaging helped catch bomb suspect

Timeline of terror hunt, from first photos to final fight 

LONDON -- British authorities ordered more police on the streets for Sunday's London Marathon in the wake of the Boston bombings, but experts warned it was "virtually impossible" to guarantee the safety of the hundreds of thousands who will attend the event. 

A police source said additional patrols by uniformed officers were planned to reassure the public in the wake of deadly attack.

While British security officials have been in contact with their counterparts in the U.S. following Monday's blasts, the U.K.'s threat level for international terrorism hasn't been changed from "substantial" -- the third of five categories on the scale.

At least 500,000 spectators are expected to watch Sunday’s race and Prince Harry is due to hand medals to the winners.

The course takes the 36,000 runners right past major sites - including Tower Bridge, the Houses of Parliament and Buckingham Palace – as well as through Canary Wharf, the giant riverside financial district targeted twice by the Irish militants in the 1990s.

Even in a city that has spent recent decades under the threat of bombs – first from Irish Republicans, more recently jihadists – such a public event poses a security headache.

Sir Bernard Hogan-Howe, the commissioner of London's Metropolitan Police, said that the force was "taking more more precautions than we might have done otherwise."

"We will make sure we've got more officers on the street looking after people, making sure they're kept safe, but we've no reason to think they'd be any less safe than before the terrible events in Boston,." he said. "We'd be professionally irresponsible if we didn't take some reasonable steps."

Sang Tan / AP

Backdropped by Buckingham Palace, a jogger crosses the Mall in London on Tuesday. It will be transformed into the finishing area for Sunday's London Marathon.

Metropolitan Police Commander Christine Jones declined to give details of what changes might be made, if any, to the event's security plan. She said officers would “continue to review all the intelligence” available.

London Marathon chief executive Nick Bitel insisted the event would go ahead. “We will be reviewing our security in the coming days, in the light of what has happened in Boston," Bitel told ITV News.

"I don't want to talk about specifics of what security we have had in the past, or will have on Sunday. All I can say is that it will be of an appropriate level to meet whatever threat assessment is made, in conjunction with the police," he added.

Hugh Robertson, a British government minister, called for crowds and runners to attend in London as normal.

“The very best way to show solidarity with Boston is to get out there on the streets of London to cheer the runners on and to show that we won’t be defeated by this sort of activity,” he told the London Evening Standard newspaper.

Runners will be encouraged to wear a black ribbon at the start of the race to honor victims of the Boston bombing, and a 30-second silence will be observed, organizers said Wednesday. 

NBC News national security analyst Michael Leiter said it was “virtually impossible” to make a marathon completely secure because of its 26.2-mile long route.

“You just have to do the best you can to keep people safe and maintain resilience," he said. “It’s important we don’t alter our lives because that provides the terrorist – domestic, international, whoever it may be – with a huge victory.”

Helmut Spahn, executive director of the International Centre for Sport Security, told Reuters: "There has to be a clear analysis of the situation and certainly no over-reaction. More police, more military is not always the best solution. To have a 100 percent security is very, very difficult if not near impossible.”

Sang Tan / AP

A sign warns of road closures linked to the forthcoming London Marathon.

This story was originally published on Wed Apr 17, 2013 2:29 AM EDT

Carl Court / AFP - Getty Images

Claire Lomas, who is paralyzed and walks with the aid of a "bionic" suit, finishes the London Marathon on May 8, 16 days after the event began. The former chiropractor was in tears as she became the first person to complete any marathon using a bionic ReWalk suit.

Kerim Okten / EPA

Claire Lomas of Britain celebrates after finishing the London Marathon, 16 days after the race began, in London, Britain, on May 8. Lomas, who is paralyzed from the waist down following a horse riding accident, is the first person to complete a marathon in a bionic ReWalk suit.

Carl Court / AFP - Getty Images

Claire Lomas, right, who is paralyzed and walks with the aid of a "bionic" suit, kisses her daughter Maisie after she finishes the London Marathon on May 8, 16 days after the event began. The former chiropractor was in tears as she became the first person to complete any marathon using a bionic ReWalk suit.

 From MSN:

A paralysed woman has become the first person to complete a marathon in a bionic suit.

Claire Lomas finished the London Marathon, crossing the finishing line 16 days after the race began.

The 32-year-old said she was "over the moon" as she completed the 26.2-mile route, which she started on April 22 with 36,000 other participants.  Read more here.

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Follow @msnbc_pictures

Jim Hollander / EPA

A marathon runner carries a Palestinian and Israeli flag as well as a a white flag with a Peace symbol as he enters the Zion Gate, after passing two Israeli Border Policemen during the Jerusalem Marathon, in Jerusalem, on March 16. It is the second year the Jerusalem Marathon has been organized and the three events, a full marathon, a half marathon and a 10 kilometers run, attracted some 15,000 participants on a chilly and rainy day in Jerusalem.

Sebastian Scheiner / AP

77-year-old Holocaust survivor Hanoch Shahar, center, runs in Jerusalem, on March 16. About 15,000 runners, including 1,500 from overseas, are competing Friday, with some 1,000 competitors expecting to complete the full 42 kilometers (26.2 miles) marathon distance, with others aiming to complete shorter distances, including Mayor Nir Barkat who says he plans to run half a marathon and 77-year old Hanoch Shahar aiming for 10km.

JERUSALEM -- Hanoch Shahar discovered a lifelong love of running as a child orphaned in World War II. On Friday, the 77-year-old Holocaust survivor ran along with some 15,000 other athletes in Jerusalem's second annual marathon.

The oldest of the runners, Shahar ran 6 miles in an hour and four minutes. He completed a full marathon two months ago and said at his age, he can run only one 26-mile race a year.

"Running gives me a sense of freedom," said Shahar, whose parents were killed by Nazis at the Theresienstadt concentration camp in what is now the Czech Republic. He said he ran there to escape his sorrow.

After the war, in a Prague orphanage, he would pass his time running and listening to track events on the radio. For hours at a time, he said, he would chase the orphanage's German Shepherd.

"That's where I got the running bug," he said.

On Friday, he and thousands of other runners dashed alongside ancient sites and through Jerusalem's steep streets in the second event of its kind in the city. The route took runners through the walled Old City, alongside the president's residence and up Mount Scopus to circle the campus of the Hebrew University of Jerusalem.

"To run through the Old City is an amazing experience," Shahar said as he boarded a bus back to his home town of Safed in northern Israel.

Read the full story.

-- Associated Press

Sebastian Scheiner / AP

Runners are seen next the Jerusalem's old city Zion Gate during the second annual marathon in Jerusalem, on March 16.