The terror suspect arrested with several knives in Whitehall on Thursday is understood to be Khalid Mohammed Omar Ali.
It is thought the 27-year-old is a British national who was born overseas and went to school in Tottenham, north London.
The suspect was detained by armed officers after a stop and search in an ‘intelligence-led’ operation.
He remains in custody, having been arrested on suspicion of offences under the Terrorism Act and possession of offensive weapons.
Police recovered knives during the swoop, which took place yards from the scene of last month’s deadly attack by Khalid Masood in Westminster.
Reports suggest the suspect was arrested after a concerned family member contacted authorities.
Detectives are continuing to question him, while two searches are being carried out at addresses in London.
It is understood the arrest was ‘intelligence-led’ and was made as part of a joint operation being carried out by MI5 and counter-terrorism police.
The Prime Minister, who was away on a campaign visit at the time, said the operation showed Britain’s spies are ‘on alert’ as she hailed the bravery of the police and the intelligence and security services.
Witnesses described the suspect as ‘very calm’ by a witness, was carrying a rucksack containing at least three knives when he was detained yards from the scene of last month’s deadly attack by Khalid Masood.
The Metropolitan Police said he was arrested ‘following a stop and search as part of an ongoing operation’ by armed officers from the specialist firearms command.
The Daily Telegraph reported that it is understood the suspect’s family had become concerned about his behaviour and reported him to the authorities several weeks ago.
Scotland Yard said in a statement: ‘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.’
Dressed in dark clothing, the suspect was wrestled to the ground on a traffic island at the Parliament Square end of Whitehall, close to the Foreign Office and the entrance to Downing Street, at approximately 2.22pm on Thursday.
He was handcuffed with his arms behind his back, one of his hands appearing to be wrapped up in a bandage as four officers stood next to him.
After being held near a red telephone box for several minutes, the suspect was led away with his black hood up and driven from the scene in a police vehicle.
Images from the scene showed three knives on the ground close to a brown rucksack, and an officer in forensic overalls could be seen taking photographs of the knives.
Westminster has been on a state of heightened alert since Masood launched his attack on March 22, killing five people, including Pc Keith Palmer.
The 52-year-old drove at pedestrians before stabbing Pc Palmer as he guarded the cobbled forecourt of the Palace of Westminster.
He was shot dead by police.
Theresa May was away from Number 10 on a campaign visit at the time of Thursday’s incident, which did not disrupt work in Downing Street, a spokeswoman said.
Mrs May said the police and security services were owed a ‘huge debt of gratitude’.
‘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,’ she said.
A French tourist, who was in New York on 9/11 and was visiting London with her daughter, saw the arrest happen.
The witness, who declined to be named, said: ‘He was very calm. I think they got him on the ground, then they stood him up against the wall.
‘There were lots of policemen. The road was blocked.
‘We didn’t stay here, we just moved because I was a bit afraid because of my daughter.’
Another witness told Sky News the suspect seemed ‘relaxed’ and said he looked like a ‘typical guy’.