LONDON (REUTERS) - British police said officers guarding one of the gates to the Westminster parliament in central London had fired a stun gun at a man who reached for a knife when they approached him on Friday (June 16) but it was not initially being considered a terrorist incident.

London Metropolitan Police said the man was acting suspiciously at around 11am local time (6pm Singapore) 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 suspect as he was restrained and bundled him into a police van, witnesses at the scene told Reuters. "The man reached for a knife," police said in a statement.

"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."

A witness at the scene told Reuters the man, who police said was in his 30s, had run towards one of the gates. "You could tell he was suspicious, he was stood there fists clenched. He looked quite an angry geezer," Bradley Allen, 19, told Reuters.

"We got seconds down the road and they had him on the floor, pinned. Police around him, telling everyone to move back."

The incident occurred less than three months since a man drove a car into pedestrians on Westminster Bridge, killing four, and then stabbed a policeman to death in the grounds of parliament. He was shot dead at the scene.

That attack was followed by a suicide bombing at a pop concert in Manchester which killed 22 and a similar deadly attack on London Bridge earlier this month, thrusting security and policing to the fore of campaigning before last Thursday's election.

The spate of recent attacks were the deadliest in Britain since four British Islamist suicide bombers killed 52 people on the London transport system in July 2005.

Another witness to Friday's incident, who asked not to be identified, said police had been shouting at crowds to get back as the man was detained in one of the busiest locations in central London which is typically crammed with tourists.

"The guy was on the ground on his front on the pavement alongside Parliament Square," the witness said. "They had him on the ground and were warning they would taze (stun) him again."

The gates to parliament were briefly closed following the incident but the area returned to normal shortly afterwards.