The incident comes a month after the March 22 attack in which a man drove into pedestrians, killing four, and stabbed a police officer to death at the gates of parliament.

"Man arrested in Westminster. No reported injuries,"Scotland Yard said on their Twitter account.

"Officers are on the scene."

David Wisniowski, who was working on a building site right next to the incident, told AFP he could see "three knives on the floor, one big one and two small".

Gabrielle Hennessy, 20, a US tourist, described seeing police rush at a man.

"We were walking along Parliament Square and we saw police rush at him. And then he was on the ground surrounded by police. They stood him up and that's when I saw the knife on the ground," she told AFP.

Police have taped off the area in question.