Armed police fired a stun gun at a man who reached for a knife as he ran towards officers guarding one of the gates to the Westminster parliament in central London this morning.
Police said the man was acting suspiciously at around 11am near one of the gates where a militant killed a policeman less than three months ago.
After firing a Taser stun gun, police detained the man.
Armed police had guns trained on the man as he was restrained and bundled into a police van, witnesses at the scene told Reuters.
"The man reached for a knife," police said. "Nobody was injured."
Although it has not declared the episode as a terrorist incident, Scotland Yard said it was "open minded as to whether terrorism was a motive".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."
A witness at the scene told Reuters the man, who police said was in his 30s, had run towards one of the gates.
The incident occurred less than three months since a man drove a car into pedestrians on Westminster Bridge, and then stabbed a policeman to death in the grounds of parliament, the first of three deadly attacks in Britain which has put the security services on high alert.
Parliament said it was aware of the incident.
"There is no reason for alarm but please continue to remain vigilant," the House of Commons, the lower house of the British parliament, said.
The gates to parliament were closed and armed police were patrolling as usual inside the perimeter, a Reuters reporter inside the building said.
On 22 March, Khalid Masood drove a car into pedestrians on Westminster Bridge, killing four people, before he ran into the grounds of parliament and stabbed a police officer to death.
He was shot dead at the scene and his attack prompted a review of security around Westminster.
