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.
An eyewitness to the arrest at Buckingham Palace told me that police walked behind the suspect and Tasered him in the back.
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.
The man's face has been blurred for legal reasons.
The Queen and Prince Philip were at Sandringham at the time of the incident.
A Buckingham Palace spokeswoman said they would not comment as it was a police matter.
A man has been Tasered by police outside Buckingham Palace, Scotland Yard said today.
A Metropolitan Police spokesman said the incident took place at 11.55am but they did not have any further information.
A man was charged today with killing another man in a nightclub.
Ashley Milne, 22, of Peckham, south east London is accused of the murder of Jamie Sanderson, 20, at the Oceana nightclub in Clarence Street, Kingston, south west London, Scotland Yard said.
He will appear in custody at South Western Magistrates' Court on Monday.
A 47-year-old man has been arrested on suspicion of murdering missing oil executive Carole Waugh, Scotland Yard said tonight. The 50 year old, originally from Durham, went missing earlier this year.
Three men have been arrested for allegedly posing as police officers and planning to steal cash and credit cards from tourists in London.
Scotland Yard said the arrests came after some visitors to the capital were approached by men claiming to be officers who then took their credit cards or cash.
Three men, aged 32, 35, and 27, were arrested in central London for impersonating a police officer and conspiracy to steal.
They were in custody in a central London police station pending further inquiries.
Detective Superintendent Steve Osborn said: "Millions of visitors to London will sadly always be an attraction to criminals and it is important that people stay alert.
"We have officers out there on the street identifying and arresting those who are involved in crime."
A man has been arrested by police investigating alleged computer hacking.
Scotland Yard said he was held at his home in north London this morning by police working in conjunction with the phone-hacking investigation.
A spokesman said: "Officers from Operation Tuleta, the investigation into criminal breaches of privacy which is being carried out in conjunction with the Metropolitan Police Service phone-hacking inquiries, arrested one man at his home in north London at approximately 6.30 this morning."
He is the seventh person to be arrested as part of Operation Tuleta.