A man was Tasered by police outside Buckingham Palace on Sunday, after he approached the building's main gates wielding two large kitchen knives and shouting.
Shocked tourists who had gathered to watch the changing of the guard looked on as the man stood outside the palace holding one of the six-inch blades to his own neck.
A video posted on YouTube showed the man take a series of swipes at one officer as the stun gun is fired before falling to the ground.
Eyewitness Kevin Burrows, 33, from Surrey, said the man broke through a cordon as the changing of the guard was about to happen.
The kitchen porter said: "I thought 'Oh my God, I can't believe it'. There must have been about 15 police officers on foot who surrounded him once he'd been Tasered.
"The police saw him really quickly and were obviously cautious. He was in his 40s I'd say and I think he had knives in both hands.
"Everybody was standing back when it happened and people were actually quite calm, I think everyone was surprised.
"They had to divert the procession away from him. I think he was making his way to the guards."
Witnesses said the band played on as the chaos unfolded.
Mr Burrows added: "The man didn't have the chance to get close to anyone as the police were right on him in seconds."
The Queen and Prince Philip were at Sandringham at the time of the incident.