A man shouting and holding knives has been tasered by police outside Buckingham Palace.
The drama unfolded as hundreds of tourists and Londoners gathered to watch the changing of the guard.
A warning - Nick Thatcher's report contains footage that some viewers may find upsetting.
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Detectives in Westminster are investigating after the man was discovered on the Route 189 bus in Marylebone shortly after midnight.
The plaque at number 16 Cowley Street in Westminster marks the brown brick Georgian townhouse where Sir John lived for 31 years.
A man reportedly armed with several knives has been arrested in Whitehall on suspicion of planning a terror attack, police have said.