A man was Tasered by police outside Parliament after he reached for a knife when approached by officers.
Scotland Yard said a man, in his 30s, was arrested on suspicion of possession of a knife.
There are no reported injuries and the incident on Friday morning is not thought to be terror related.
A bearded man wearing a grey hooded jumper and black trousers was pictured being held against Parliament’s railings with his arms behind his back by a police officer.
The Carriage Gates entrance to Parliament was closed following the arrest but has since been re-opened.
Ben Glaze, the Daily Mirror’s deputy political editor who witnessed the incident, told Sky News: “I was in the Mirror’s office in Parliament, which overlooks New Palace Yard, when we heard some shouting.
“We looked out of the window and saw armed police running across New Palace Yard towards the exits.
“There was lots of shouting, a bit of confusion, Carriage Gates were quickly closed and now there are armed police surrounding the chap who was on the floor.”
Mr Glaze said police wearing blue plastic gloves appeared to search the man and picked up items off the floor before putting them in plastic bags. They also removed the man’s belt and put it in an evidence bag.
A police riot van, an armed response vehicle and an unmarked police vehicle were seen attending the scene.
A Parliamentary spokesman said: “We are aware of the earlier incident outside Carriage Gates. An individual has been arrested.
“The incident has now ended and access to the estate has returned to normal.”
On 22 March, PC Keith Palmer was stabbed to death just inside Parliament by terror attacker Khalid Masood, who was gunned down by armed police.
Masood entered Parliament’s New Palace Yard through Carriage Gates. Following the attack the entrance’s gates were replaced by bulkier, less decorative gates with wire meshing.
Commons Speaker John Bercow ordered a review of Parliament’s security in the wake of PC Palmer’s death.