A 41-year-old man has been arrested in the grounds of Buckingham Palace after scaling a perimeter wall.
The suspect was in the grounds for seven minutes before being arrested on Wednesday night, police said.
The Queen and the Duke of Edinburgh were inside at the time and have been informed of the security breach.
The man was held on suspicion of trespassing on a protected site shortly before 9pm.
He had climbed the outside perimeter wall to enter the grounds, leading the alarm to be raised. No weapons were found.
Commander Adrian Usher, head of the Met's Royalty and Specialist Protection unit, 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."
Former royal protection officer Dai Davies said that the arrest came "very quickly" after the alarm was raised, considering the size of the grounds.
He said: "He was caught by the police very quickly and the system seems to have worked, which is the main thing."
The Queen had earlier attended the State Opening of Parliament and was today attending a garden party in the grounds of the Palace.
A number of trespasses have taken place at the site in recent years, and the walls have been scaled several times.
Fathers' rights campaigners used a ladder to climb on the roof of the Queen's Gallery in November and spent almost eight hours on top.
In September 2013, a 37-year-old man got into the Palace building itself. He was found in a room which was open to the public during that day.
The most high-profile breach came when Michael Fagan climbed over palace walls and broke into the Queen's bedroom in 1982. He spent around 10 minutes talking to her.