A man armed with a "backpack of knives" has been arrested close to the Houses of Parliament.
Metropolitan Police confirm one suspect has been detained following an incident in Whitehall in central London, but no injuries have been reported. The suspect was detained following a stop and search as part of an "ongoing operation".
Pictures taken at the scene show armed police surrounding a man close to Parliament and the official residence of Prime Minister Theresa May.
May's spokesperson said the prime minister was not inside Downing Street at the time of the incident.
Transport for London (TfL) advised that the roads on Whitehall / Parliament Street are currently closed northbound due to a police incident and to expect long delays.
The incident occurred close to the attack on Westminster Bridge, outside the Houses of Parliament, on 22 March.
Terrorist Khalid Masood was shot dead by police marksmen after killing five people, including a police officer, after he ploughed through pedestrians on Westminster Bridge before crashing the car vehicle outside the gates of Parliament and stabbing PC Keith Palmer.
Police said there is no information to suggest a wider threat.
A Met Police spokesperson said: "A man has been arrested in Whitehall this afternoon [27 April], at approximately 2:22pm following a stop and search as part of an ongoing operation.
"The 27-year-old man was arrested in Parliament Street, junction with Parliament Square, by armed officers from the Met's Specialist Firearms Command.
"He 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."
Mayor of London Sadiq Khan added: "Thanks to all officers who dealt swiftly and professionally with the incident in Whitehall. The Met Commissioner has briefed me on the arrest."