A 21-year-old man has been arrested after allegedly climbing over a gate at Buckingham Palace.
Police were alerted to a man scaling the south gate of the royal residence at 1.45pm on Friday.
Officers met the man as he dropped to the ground and arrested him for trespassing on a designated site, Scotland Yard said.
The man was not carrying an offensive weapon and has been taken into custody.
He will undergo a mental health assessment and inquires are continuing, police added.
It is understood that no members of the Royal Family were in residence at the time of the security breach.
The Queen had undertaken an investiture ceremony at Windsor Castle earlier in the day.
Her London home has long been a target for intrusions - with security breaches at Buckingham Palace ranging from a naked paraglider landing on the roof to an undercover journalist posing as a footman.
The most notorious was when unemployed decorator Michael Fagan made his way over the walls and up a drainpipe, and wandered into the Queen's bedroom in 1982. She woke to find Fagan sitting on her bed.