A man was brought down with a Taser outside the Palace of Westminster after he reached for a knife when approached by officers, Scotland Yard has said.
A bearded man, wearing black trousers and a grey sweatshirt, was seen being held against the fence with his arms behind his back by a police officer.
Police said the man was arrested on suspicion of possessing a knife.
But they “remain open minded as to whether terrorism was a motive”, they said.
The man was held just yards from where PC Keith Palmer was stabbed to death by terrorist Khalid Masood in March.
Joe Murphy, London Evening Standard political editor, tweeted: “There were shouts of ‘knife, knife, knife’ and man was brought down with a Taser.”
BBC deputy political editor John Pienaar said the latest incident ended very swiftly with the man being taken away in a police van.
While the gates are closed, the road and pavement opposite remained open and the incident had no effect on traffic in Parliament Square, he said.
In a statement, the Metropolitan Police said “it is too early to understand the motivation” of the man “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.”
Source: BBC News