Police have arrested a man who was reportedly carrying a backpack of knives in Whitehall.
Scotland Yard said a man had been arrested near Downing Street.
Mirror Online reports the arrested man was carrying a "backpack of knives", close to Parliament Square.
Eyewitnesses said armed police officers detained the suspect.
Scotland Yard said there are no reported injuries.
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Further pictures show forensics at the scene
Mayor of London thanks police
Picture appears to show arrested man
Picture shows officers at the scene
'No immediate threat'
Theresa May was 'not at home'
British PM Theresa May was not at Downing Street during the incident, her office spokesman has just confirmed.
As reported earlier, this incident was just yards from No.10.
At least 10 officers and six police cars swarmed on Whitehall as the incident unfolded.
Man detained under Terrorism Act
A Met police spokesman said: “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 twenties, 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.”