A 41-year-old man has been arrested at Buckingham Palace following a security breach.
The suspect was arrested after scaling an outside perimeter wall on Wednesday evening, police said.
He was found on the grounds by police and is being held on suspicion of trespassing on a protected site.
Commander Adrian Usher, head of the Met's Royalty and Specialist Protection, said: "I am content that our security measures worked effectively on this occasion and at no time was any individual at risk."
A Palace spokesman said: "We never comment on security, which is a matter for the police."
The man was in the palace grounds for seven minutes after the alarm was raised, Scotland Yard said.
The Queen is likely to have been at home in the palace last night after attending the State Opening of Parliament.
She will be at a garden party there on Thursday afternoon.
No weapons were found on the man when police arrested him shortly before 9pm.