A man has been arrested in the Royal Mews area next to Buckingham Palace following reports a person was seen climbing a wall into the area.
A 25-year-old man was detained outside the stables at about 01:25 BST and held under the Serious Organised Crime and Police Act on suspicion of trespassing.
The stables, home to the historic royal carriages, were not entered by an intruder, Scotland Yard said.
The man was taken to a London police station for questioning.
The force added “at no point” did the intruder “enter Buckingham Palace or the Palace Gardens”.