A man carrying knives near the Houses of Parliament has been wrestled to the ground by armed police and arrested on suspicion of terrorism offences.
The man, aged 27, was detained as part of an intelligence-led operation on Parliament Street.
A witness described seeing two knives on the ground, one of which he described as a large bread knife.
The Metropolitan Police said there were no injuries. The suspect is in custody in a south London police station.
A French tourist who saw the arrest said the suspect looked "very calm". Another witness said the man was not shouting, or acting aggressively.
A police statement said: "The man 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.
"Detectives from the counter-terrorism command are continuing their investigation, and as a result of this arrest there is no immediate known threat."
The incident happened just yards from Downing Street, although Prime Minister Theresa May spent the day campaigning in Derbyshire and Leeds.
Mrs May said: "I think it shows that our police and our intelligence and security services are on the alert, as they always are, looking to keep us safe and secure."
The BBC's home affairs correspondent June Kelly said: "It's understood that the man was detained as part of an ongoing operation by Scotland Yard's counter-terrorism command.
"Officers working on intelligence moved in on him in Whitehall. He was then searched and arrested."
It's thought the man lives in London and holds a British passport, but was born outside the UK. Addresses linked to him are being searched.
Eyewitness Helen Chandler-Wilde described seeing "knives and a brown backpack lying on the floor" at the scene at the time of the arrest.
She told BBC Radio 5 Live that passers-by remained calm and were "remarkably not scared at all".
by BBC security correspondent Frank Gardner
This, on the face of it, appears to have been a successful intelligence-led operation by the Metropolitan Police and MI5, the security service.
The suspect arrested today had been under surveillance for some time.
It's not known what his exact intentions were. But the fact that he was arrested with two knives, close to Parliament and the centre of government, has contributed to this case being investigated by SO15, the Met Police's counter-terrorism command.
At this stage it is not thought that any other country's law enforcement agency is involved.
The area around Whitehall has been on increased alert since 22 March when Khalid Masood killed five people, including a police officer, in a terrorist attack.
In addition to visible armed officers, a number of armed police officers regularly patrol the streets of the capital with mobile armouries, ready to respond to an attack.
At the scene
by Mario Cacciottolo, BBC News
A number of items were left on the ground following the incident in Whitehall.
Two police forensics officers, covered in crime scene suits, slowly photographed and picked each one up.
Several knives and what looked like paper or tissue were all put into a separate bag or box. When it came to the brown rucksack lying on the floor, the officer opened it up somewhat gingerly.
Their every move was followed by the click, click, click of camera lenses, both from the press and tourists.