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A girl holds a sign reading "People want a no fly zone" during a demonstration against Syrian President Bashar al-Assad in Homs on Tuesday.

UPDATED: 5 p.m. ET

HOMS, Syria -- The presence of Arab League monitors in Syria has re-energized the anti-government protest movement, with tens of thousands turning out over the past three days in cities and neighborhoods where the observers are expected to visit. The huge rallies have been met by lethal gunfire from security forces apparently worried about multiple mass sit-ins modeled after Cairo's Tahrir Square.

 On Thursday, security forces opened fire on tens of thousands protesting outside a mosque in a Damascus suburb and killed at least four. The crowd had gathered at the mosque near to a municipal building where cars of the monitors had been spotted outside.

Much of the bloodshed of the past few days appeared to be a desperate attempt by authorities to keep protesters from gaining ground for multiple mass sit-ins where they can recreate the model of Cairo's Tahrir Square. The two-week sit-in at Tahrir brought down longtime authoritarian ruler Hosni Mubarak in February and inspired other uprisings across the Arab world.

On Tuesday, as monitors visited the flashpoint city of Homs in central Syria, troops shot at thousands of protesters trying to reach the city's central Clock Square. On Wednesday, the scene was repeated in nearby Hama, where protesters were shot trying to reach Assi Square and activists said at least six people were killed.

"This is the regime's biggest fear, to have hundreds of thousands of people gathered in one place," said one Homs resident.

Syria has allowed the monitors in, released about 800 prisoners and pulled some of its tanks from the city of Homs. But it has continued to shoot and kill unarmed protesters and has not lived up to any other terms of the agreement.

Syria's top opposition leader, Burhan Ghalioun, told reporters in Cairo after meeting Arab League Chief Nabil Elaraby that the aim of the mission is not only to observe, but to make sure that the Syrian government is "stopping the killing and shooting." He added that the Syrian government is holding more than 100,000 detainees, "some of them held in military barracks and aboard ships off the Syrian coast." He added: "There is real danger that the regime might kill them to say there are no prisoners."

State-run TV said monitors also visited the Damascus suburb of Harasta, the central city of Hama and the southern province of Daraa, where the uprising against Assad began in March.

The Observatory said a total of 26 people have been shot by security forces and killed on Thursday, most of them in several suburbs of Damascus. The Local Coordination Committees, another activist group, said 35 people were killed. The differing death tolls could not be immediately reconciled as Syria bans most foreign journalists and keeps tight restrictions on the local media.

AFP - Getty Images

Syrian army tanks in the background as a group of Arab League observers tour the flashpoint central city of Homs on December 27, 2011.

The Syrian government organized a tour to the restive central city of Homs, where one team of monitors has been working for the last three days.

At the entrance to the city, which witnessed much of the violence in the past months, two checkpoints were stopping cars and asking for people's identity cards. Inside, most shops were closed and streets had few people and cars as sporadic gunfire rang out. Most main streets were clean, but side streets were lined with piles of garbage bags.

At the military hospital, one of the largest in the city, a large number of civilians and members of the military were receiving treatment. One of them was a soldier who was shot in the stomach while in a Homs street Thursday morning. He was undergoing an operation, his mother said.

"My son did not harm anyone. He is a soldier to protect the country," said his mother, Zeinab Jaroud, as she stood holding back here tears outside the operating room.

Troops fired live ammunition and tear gas to disperse large protests in several areas of the country, including central Damascus, killing at least 26 people nationwide, activists said. A key activist network, the Local Coordination Committees, said it has documented the names of 130 people, including six children, who died since the Arab League monitors arrived in Syria Monday night.

The ongoing violence, and new questions about the human rights record of the head of the Arab League monitors, are reinforcing the opposition's view that Syria's limited cooperation with the observers is nothing more than a farce for President Bashar Assad's regime to buy time and forestall more international condemnation and sanctions.

Still, the presence of outside monitors has invigorated frustrated protesters and motivated them to take to the streets again in large numbers after months of demonstrations met by bullets had dashed their hopes of peaceful change.

"We know the observers won't do anything to help us," said Yahya Abdel-Bari, an activist in the Damascus suburb of Douma. "But still, we want to show them our numbers, to let them know what is really happening here," he said.

The 60 Arab League monitors, who began work Tuesday, are the first Syria has allowed in during the nine-month anti-government uprising. They are supposed to ensure the regime complies with terms of the Arab League plan to end Assad's crackdown on dissent. The U.N. says more than 5,000 people have died in the uprising since March.

The plan, which Syria agreed to on Dec. 19, demands that the government remove its security forces and heavy weapons from cities, start talks with the opposition and allow human rights workers and journalists into the country. It also calls for the release of all political prisoners.

As word spread Thursday morning that the observers would be visiting Douma — which saw an intense government crackdown in the early days of the uprising — thousands of people began gathering outside the Grand Mosque, calling for Assad's downfall and for international protection for civilians.

Amateur videos posted on the Internet showed protesters in Douma facing off with Syrian soldiers, shouting "Freedom, Freedom!" Troops then opened fire to disperse the protesters, whose numbers had swelled to around 20,000.

"It came like rain, they used heavy machine guns, Kalashnikovs, everything," said Abdel-Bari.

Four people were killed and scores others wounded, said Abdel-Bari and various activist groups.

Rami Abdul-Rahman, head of the British-based Syrian Observatory for Human Rights, said cars belonging to the Arab League monitors were seen in front of a municipal building close to the mosque around the same time.

But after the killings, Abdul-Rahman and Abdel-Bari said the monitors were barred by security officials from entering Douma and the situation quickly deteriorated. A witness said angry citizens closed off streets with rocks and garbage containers and thousands of people returned to the area around the Grand Mosque to stage a sit-in.

Troops also surrounded a mosque in Damascus' central neighborhood of Midan and tossed tear gas canisters at hundreds of people calling for the downfall of the regime.

In the northern Idlib province, some 150,000 protesters took to the streets — more than on any other day recently, the Observatory said.

"The presence of monitors is a source of comfort to the Syrian street and breaks the barrier of fear for those who were hesitant about protesting," said Abdul-Rahman.

Although the violence against protesters has not stopped, he said the death toll would have probably been double what it is had there been no monitors on the ground.