A man has been Tasered by police outside Buckingham Palace.
A Buckingham Palace spokeswoman said they would not comment as it was a police matter.
The British monarch, Queen Elizabeth, and her husband Prince Philip were at their country estate in 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 50s. He was arrested on suspicion of affray and has been taken to a central London police station."