A man has been arrested in the Royal Mews area adjacent to Buckingham Palace.
Scotland Yard said officers at the royal residence responded to a person climbing the wall and entering the Royal Mews at about 1.25am on Saturday.
Following a search, a 25-year-old man was detained outside the stables.
He has been arrested under the Serious Organised Crime and Police Act on suspicion of trespassing on a protected site and has been taken to a London police station.
The force added: “At no point did the man enter Buckingham Palace or the Palace Gardens.”
