A man has been arrested after an incident in Whitehall, Scotland Yard has said.
There were unconfirmed reports that knives were recovered from the scene, close to Parliament Square.
Eyewitnesses said armed police officers detained the suspect.
Scotland Yard said there had been no reported injuries.
Police appear to have made the arrest close to the junction of Parliament Street and Westminster Bridge Road.
An handful of armed and unarmed officers were at the scene which is yards from where Khalid Masood launched a terror attack in March which killed five, including PC Keith Palmer.
The officer was stabbed to death as he carried out his duties on the cobbled forecourt of the Palace of Westminster.
Pictures posted by people near the scene showed a man pinned to the ground next to a bag, which some witnesses had said contained knives.
An employee at HMRC said the main entrance to their building at 100 Parliament Street has been closed.