Armed British police officers arrested a man carrying knives near Prime Minister Theresa May's office in London on Thursday on suspicion of preparing an act of terrorism.
The 27-year-old man was detained by counter-terrorism officers on Parliament Street, a stone's throw from the parliament building, May's Downing Street office and government departments, during a stop-and-search in an ongoing security operation, police said.
Dramatic pictures of the man's arrest on social media showed a large group of heavily armed officers surrounding the suspect, and a Reuters photographer saw three knives and a rucksack lying on the ground.
Police said the suspect was being questioned at a south London police station.
No one was injured in the incident, and May was not at Downing Street at the time because she was campaigning in northern England for a June 8 parliamentary election.
Two hours after he was detained, police reopened streets in the Westminster district, indicating they saw no further public risk.
TRT World's Sarah Morice has more on the story.
No immediate threat
A Western security source said the man, from London, was on the radar of counter-terrorism officers and the domestic intelligence agency MI5 before his arrest.
"Detectives from the Counter Terrorism Command are continuing their investigation, and as a result of this arrest there is no immediate known threat," police said in a statement.
"The man ... was arrested on suspicion of possession of an offensive weapon and on suspicion of the commission, preparation and instigation of acts of terrorism. Knives have been recovered from him."
Since August 2014, Britain has been on its second-highest alert level of "severe", meaning an attack by militants is considered highly likely.
Just weeks before the attack in Westminster, London's counter-terrorism police chief said the security services had thwarted 13 terrorism attempts since May 2013, with hundreds of investigations ongoing at any time.