Counter-terror detectives have launched an investigation after a man was Tasered outside the Palace of Westminster when he allegedly reached for a knife.
Police have not declared the episode a terrorist incident but say they are "open-minded as to whether terrorism was a motive".
The bearded suspect, in his 30s, was arrested on suspicion of possession of a knife after being approached by officers near the Houses of Parliament.
The exchange unfolded yards from the scene of the Westminster terrorist attack in March and comes amid heightened security after the Manchester and London Bridge atrocities.
Scotland Yard said: "At this time it is too early to understand the motivation so we have not declared this a terrorist incident.
"However, given the location, the circumstances and recent tragic events, the MPS counter-terrorism command will be investigating this incident.
"We remain open-minded as to whether terrorism was a motive."
At 11.10am on Friday, officers on routine duties near the Carriage Gates entrance to the Palace of Westminster became aware of a man acting suspiciously, the Metropolitan Police said.
"The officers approached the man in order to speak with him," the force added. "The man reached for a knife, and police discharged a Taser."
The suspect, wearing black trousers and a grey sweatshirt, was held with his arms against his back close to the forecourt where Pc Keith Palmer was stabbed to death by terrorist Khalid Masood in March.
He was led away into a van by officers after being detained for around 30 minutes, before being taken to a central London police station, where he remains in custody.
Eyewitness Bradley Allen, 19, from Barking, east London, said he saw the man with clenched fists moments before he was detained.
Mr Allen said he was walking past Parliament when he saw a "dark male stood there, who looked quite angry with fists quite clenched".
"He stared at me and my mate and we walked past him. Within seconds there was a shout and police ran out of the front gate shouting at him 'Get on the floor, get on the ground'.
"Within seconds he was on the ground - I'm not sure whether he was Tasered - I'm not sure how he got down there, but in seconds it was over.
"Armed police everywhere, telling everyone to move back, and we all got pushed back."
No one was injured in the incident.
A review of security at the perimeter of the parliamentary estate was ordered following Masood's car and knife rampage, which claimed the lives of five victims, including Pc Palmer.
The use of armed officers and physical barriers has become more visible, while a permanent barrier has been erected to protect pedestrians on Westminster Bridge.