Man detained after Westminster incident was known to MI5 - security source

LONDON (Reuters) - The London man detained by armed police in Westminster was known to counter-terrorism officers and Britain's domestic intelligence agency MI5 before his arrest, a Western security source told Reuters.

The 27-year-old man was detained by counter-terrorism officers on Parliament Street, a stone's throw from the UK parliament building and May's Downing Street office, during a stop and search as part of an ongoing operation, London police said.

(Reporting by Michael Holden; editing by Guy Faulconbridge)