Police arrest 'terror' knifeman outside British parliament
Scotland Yard said the 27-year-old man was arrested at 13:22 pm following a "stop and search" procedure as part of an ongoing operation.
The man was then pinned to the ground in the middle of the road close to the Treasury and entrances to Westminster Underground station.
He was arrested on suspicion of "possession of an offensive weapon" and on suspicion of the "commission, preparation and instigation of acts of terrorism," police said.
Firearms officers were present during the incident.
"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.
Nobody was injured in the incident, London's Scotland Yard police headquarters said.
The incident took place in full view of crowds of tourists posing for photographs with the Houses of Parliament. The suspect was described as bearded with close-cropped hair.
Gabrielle Hennessy, 20, a US tourist, described seeing police rush at a man.
"We were walking along Parliament Square and we saw police rush at him. And then he was on the ground surrounded by police. They stood him up and that's when I saw the knife on the ground," she told AFP.
The incident took place close to Downing Street, although Prime Minister Theresa May was outside of London on a general election campaign at the time.
The area has been on a state of heightened alert since an attack on March 22 perpetrated by Khalid Masood, a 52-year-old convert to Islam.
Masood drove into pedestrians, killing four, and stabbed a police officer to death at the gates of parliament before being shot dead by an armed officer.