A man holding a knife to his neck was immobilized with a stun gun by police outside London’s Buckingham Palace during Sunday’s Changing of the Guard ceremony.

Smartphone video of the incident, published on news sites including the Daily Telegraph, showed the man being brought to the ground by the police weapon.

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He was immediately arrested and is in custody, according to a Metropolitan Police spokesman, although it is not yet clear if he will be charged with any offense.

The incident happened at 11:55 a.m. local time (6:55 a.m. ET) on the street outside the palace gates in the west end of central London.

It was witnessed by hundreds of tourists who had gathered to see the 45-minute Changing of the Guard ceremony.

The queen and her husband, Prince Philip, were at their rural home in Sandringham, Norfolk, at the time.

Buckingham Palace said it would not comment, according to ITV News.

Updated at 11:22 a.m. ET: LONDON -- Two British Olympians said Wednesday that their medals were stolen -- and they've turned to social media in an attempt to get them back.

British rower Alex Partridge and field hockey player Hannah Macleod said their 2012 Olympic bronze medals, along with Partridge’s "Team GB" blazer, disappeared early Wednesday at an upmarket London nightclub. They were among a group of athletes who went to the club after attending a royal reception at Buckingham Palace to celebrate the British team's success at the 2012 Games.

Under the Twitter name @2012parters, Partridge, 31, posted a CCTV image with the tweet: “Does anyone recognize this man as he may be able to help me get my @London2012 bronze medal and @TeamGB blazer back.”

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Macleod, 28, tweeted: “My medal was also stolen at the same time. Totally devestated (sic).” 

Home advantage: Britain celebrates 'sensational' Olympic medal haul

With the name @hannahmacleod6, she also posted the CCTV image, adding, “This man MIGHT be able to shed some light on the theft ... I'm not after punishment. If you picked up a Bronze Olympic medal that isn't yours pls just send anonymously back to GB hockey-Bisham Abbey.”

In an interview Wednesday night on BBC Radio 5 Live, Partridge said he initially thought his possessions had been mistakenly taken by another team member. But after looking at a CCTV recording, he said that doesn't appear to be the case.

“Someone’s taken my Olympic blazer with my Olympic bronze medal in it and all I want is to get them both back," Partridge said.

The full impact of the incident only hit him when he got home, he said. “I suddenly realized my daughter would never see this medal -- what I’d worked so hard for, for 12 years.”

Police issued an appeal for information into the alleged theft and said the missing jacket, but neither of the medals, had been handed into police Thursday morning.

More London 2012 coverage from NBC News

"My team have spoken to both Olympians who are understandably desperate to be reunited with the medals they have dedicated their careers to win," Detective Chief Inspector James Harman said. 

A 29-year-old man had been arrested in connection with the investigation and was being held at a West London police station, police said.

 Phelps, Rupp, Pistorius: Emotional moments

Dozens of British Olympic and Paralympic medalists had been invited to the palace Tuesday for an event attended by the queen, Prince Philip and the Duchess of Cambridge.

Read more on this story on NBC News' British partner ITV News

The team’s formal uniform included special inside pockets to hold their medals, according to ITV News. 

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