British armed police detained a man on suspicion of having a knife after he ran, shouting, towards one of the gates of the Westminster parliament in central London on Friday.
Metropolitan Police said officers on routine duties near the entrance to the Palace of Westminster became aware of a man acting suspiciously.
"The officers approached the man in order to speak with him. The man reached for a knife, and police discharged a Taser," it said in a statement.
The man in his 30s was arrested for possession of a knife.
"Nobody was injured," police added.
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The arrested man has been taken to a central London police station, where he remains in custody.
A witness at the scene told Reuters the man ran towards one of the gates to parliament where a militant killed a policeman less than three months ago.
Another witness told Reuters he saw police threatening to use a stun gun on the man. Pictures from the scene showed the man on the ground with an officer pointing a gun at him.
Officers later put the man in the back of a police van, a Reuters reporter said.
Parliament said it was aware of the incident.
The gates to parliament were closed and armed police were patrolling as usual inside the perimeter, a Reuters reporter inside the building said.
On March 22, Khalid Masood drove a car into pedestrians on Westminster Bridge, and then stabbed a policeman to death in the grounds of parliament.
A suicide bombing in Manchester and a deadly attack on London Bridge thrust security and policing to the fore of the campaign 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.
