A terror suspect arrested near the Houses of Parliament has been named as Khalid Mohamed Omar Ali.
The 27-year-old is believed to be a British national who was born overseas and went to school in Tottenham, north London.
He was known to police prior to his arrest, according to Sky sources.
Police say the suspect was detained by armed officers just yards from Downing Street after a stop and search in an "intelligence-led" operation.
He was carrying a rucksack containing several knives, which are being examined by forensic officers.
Roads were locked down as officers carried out the arrest, pushing the man to the ground before holding him at the scene.
He was taken to a south London police station where he is being held under the Terrorism Act.
The arrest took place at the Parliament Street junction with Parliament Square, close to the scene of last month's terror attack which killed five people, including police officer Keith Palmer.
A witness said the suspect was "very calm" as he was arrested, and did not struggle as he was held on the ground.
A builder working nearby said the man was not waving knives, acting aggressively or shouting.
He said: "I saw the police officers catch him. They caught him and got him on the ground."
Theresa May was not at No 10 at the time of the terror alert.
Speaking from a General Election campaign event in Derbyshire, the Prime Minister said: "I'm aware that a man has been arrested in Whitehall today and that individual has been arrested on the basis of a terrorism charge.
"I think it shows that our police and intelligence and security services are on alert as they always are - looking to keep us safe and secure. I would say we owe a huge debt of gratitude to these people."
The arrest was unconnected to anti-terror raids in northwest London and Kent on Thursday evening, in which a woman was shot by police and six people were arrested.