A taser gun was used on a man brandishing a knife to his neck outside Buckingham Palace during the Changing of the Guard ceremony on Sunday 3 February.
Large crowds had gathered to witness the Changing of the Guard as the man appeared, holding a knife to his throat. Witnesses said the band played on as the chaos unfolded.
Within ten seconds, police arrived on the scene and fired a Taser at the man who, after jabbing a knife at a police officer, collapsed.
Kevin Burrows, 33, a kitchen porter from Kingston, said: "A man just started running towards the palace, I just saw him running towards the front gate. I think he had three knives.
"Then the police suddenly surrounded him. They were shouting at everybody to get back. There were police on horses. I saw the shot fired, the police officer was about three metres from him."
Josh Greenberg, who was at the scene said: "He was shouting but it was quite unintelligible. Police were clearly telling him to drop the knife and instructing the public to get back."
The man, who is understood to be in his 50s, was taken to a central London police station for further questioning and a medical examination.
Witness Grant Shepherd posted a video of the incident on YouTube after tweeting: "Crazy man with a knife to his throat just stopped the Changing of the Guard at Buckingham Palace."
In the YouTube video, the man takes a series of swipes at an officer as the stun gun is fired.
As the Taser connects, he falls to the ground, watched by hundreds of tourists.
A spokesman for the Metropolitan Police said: "A man was seen outside the centre gate in possession of two knives. He was not making threats to members of the public but he was challenged by police.
"He acted aggressively and a Taser was discharged. He is thought to be in his fifties. He was arrested on suspicion of affray and has been taken to a central London police station."
According to the latest statistics from Scotland Yard, the Metropolitan Police used a Taser 224 times between October 1 2010 and March 31 2011.
Watch the incident unfold on the YouTube video below.