The suspect in the latest incident was seen pinned to the ground after his arrest, with armed officers standing over him. Shortly afterwards forensic experts could be seen examining the ground where at least one knife had fallen onto the road.

The man, believed to be from the London area, had come to the attention of MI5 and Scotland Yard’s counter-terrorism command.

Investigators believe he may have been about to launch an attack and he was stopped by armed officers in a “targeted” stop and search.

The suspect is believed to have been acting alone, and the investigation will try to establish what his targets may have been.

Given the location of the arrest investigators believe one possibility was an attack on the Westminster or Whitehall areas. Past Islamist propaganda has called for attacks on civilians or symbols of power.

The arrest of the man came in an area rich with government buildings and tourists, with Downing Street yards away. The prime minister, Theresa May, was not in her official residence at the time.

Police said the arrest came at the junction of Parliament Street, and Parliament Square. It is barely 30 yards from Westminster Bridge, where last month a terrorist attack killed civilians, and the parliamentary gate near which PC Keith Palmer was fatally stabbed.

Scotland Yard’s counter-terrorism command was leading the investigation into the man and why he was allegedly in possession of knives.

Police said in a statement giving the first official details: “A man has been arrested in Whitehall this afternoon, at approximately 14.22, following a stop and search as part of an ongoing operation.

“The man was arrested in Parliament Street, junction with Parliament Square, by armed officers from the Met’s specialist firearms command.

“The man, aged in his late 20s, was arrested on suspicion of possession of an offensive weapon and on suspicion of the commission, preparation and instigation of acts of terrorism. Knives have been recovered from him.

“He is being detained under the Terrorism Act and is in custody in a south London police station.

“Detectives from the counter-terrorism command are continuing their investigation, and as a result of this arrest there is no immediate known threat.”

Security is especially tight around Whitehall following last month’s terrorist attack in Westminster.

The suspect was held on Parliament Street for more than half an hour, without speaking or struggling. While two officers detained him, around two dozen heavily armed police blocked off the traffic and pedestrians on one side of the road with tape.

He was led away into a police car, while scenes of crime officers examined knives and a rucksack on the traffic island in the middle of Parliament Street at the junction with Parliament Square.

Few MPs were in the Palace of Westminster because parliament is winding down before the election and Theresa May has already headed out on to the campaign trail.

However, civil servants were still working as normal, with a number of HMRC staff shut outside their building, as the Whitehall entrance was closed off. The atmosphere in parliament remained calm and all gates were operating as normal during the incident.

No injuries have been reported after the incident.

The terrorism threat level for the United Kingdom remains at severe, meaning an attack by Islamist terrorists is assessed to be highly likely.