A man has been Tasered by police outside Buckingham Palace, Scotland Yard said on Sunday.
A Metropolitan Police spokesman said the incident took place at 11.50am.
The Queen and Prince Philip were at Sandringham at the time of the incident.
A spokesman for the Metropolitan Police said: "It was approximately 11.50. 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."
A scene was cordoned off with police tape 30 metres from the palace gates this morning and an ambulance was parked to the side of the building.
Inside the cordoned-off area lay two knives with six-inch blades and a pair of Nike high top trainers.
Taser wire also lay on the floor with four plastic tubes of two different sizes, three large ones and a small one.
A hat and a black carrier bag containing paper material with a handkerchief beside it also lay on the ground.
Police recovered two knives from the scene
Police at the scene were telling members of the public that a man "tried to make an exhibition of himself during the guard change" and that there were "mental health issues".
The four tubes inside the cordon were police property used for removing "sharps".
A video on YouTube shows the man putting a blade to his own neck and shouting before police shoot him with the Taser.
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."