Asuspected jihadist carrying a rucksack packed with knives was heading towards Downing Street when he was dramatically arrested by armed police, security sources have told The Telegraph.
The ‘lone wolf’ suspect had been under close surveillance when counter-terrorism officers ordered his immediate detention as he came within 300 yards of the gates of the Prime Minister’s residence.
It is understood that the suspect’s family had become concerned about his behaviour and reported him to the authorities several weeks ago. Investigators believe he was about to launch an attack.
The 27-year-old man, dressed head to toe in black, was wrestled to the ground by armed police officers as he walked among a crowd of tourists and workers in Parliament Street at just after 2.20pm on Thursday.
His arrest came just five weeks - and a 100 yards away - from the spot where Khalid Masood was shot and killed within the grounds of the Palace of Westminster, having mowed down bystanders in a hired car driven over Westminster Bridge.
Theresa May, who was on a tour of a factory in Chesterfield at the time, praised the police and security services for preventing another terrorist strike at the heart of British power.
Mrs May said: “It shows that our police and intelligence and security services are on the alert, as they always are, looking to keep us safe and secure.
“We owe a huge debt of gratitude to these people, many of whom are unseen, unheard, yet the job they do day in, day out to keep us secure is a really important one and we should thank them for it.”
A Whitehall source told The Telegraph that the man, who is understood to have been born overseas but who lives in south east London, had been arrested as part on an “intelligence-led operation”. he had been followed on the Underground and is thought to have surfaced at St James’s Park tube station before heading towards Westminster.
The source said: “He was under surveillance and he was stopped while walking away from Parliament Square and up Whitehall towards Downing Street.”
A senior policing source said: “This is someone who was known and under surveillance. He was being watched very closely. He had been travelling on the London Underground and we understand he had exited the Tube at St James’s Park station before walking in the direction of Whitehall and Number 10.
“At some stage an alert went out to all the teams in the area and the order was made to pick him up.”
Police said that as a result of the arrest there “is no immediate known threat”, suggesting there is no evidence the man was a member of a terror network but was likely acting as a ‘lone wolf’.
Photographs taken during and after the dramatic arrest showed a bearded man, possibly of Asian or east African origin with cropped black hair, held at the scene for at least ten minutes. In one image, he appeared to be smirking for the camera.
One of his hands appeared to be wrapped up in a bandage as four officers stood next to him.
On the ground lay a cheap rucksack, with the Union flag and the word ‘London’ emblazoned on it. A number of knives lay scattered on the ground. One appeared to be a large kitchen knife and another a flick-knife.
Two police forensic officers, wearing crime scene suits, combed through the evidence. They took photographs of the contents of the bag, before taking them away for further examination.
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 ‘gingerly’.
It is not clear if police knew he was carrying weapons when the decision was made to stop him.
It was being suggested he had reached into his bag when police acted but other sources said this was not the case.
The Telegraph has been told the arrest was ordered because he was heading towards Downing Street, rather than any specific intelligence about the contents of the backpack. The man was on crossing of the central reservation of the busy street when armed officers moved in.
Eyewitnesses told of the dramatic swoop in broad daylight.
Ian Grant, 56, from Poplar in east London, had just emerged from the Westminster tube station when he saw “a police car go flying by”.
Mr Grant said: “All of a sudden two cops got out. Then an armed cop with a rifle came past me. I thought it was just a drill at first. Then he shouted ‘Armed police, stop’ and they got this guy down on the ground.
“When they shouted at him he didn’t resist or say a word, he just put his hands up and stopped. Then they got the cuffs on him in the central reservation.”
Mr Grant went on: “First there was one, then two police officers, then they all came from nowhere like a swarm of bees. There was about a dozen cop cars.”
Mr Grant said: “Before he was arrested he was just walking along normally, but he must have known they were after him because he didn’t say anything when they stopped him.”
Paul Morris, 29, who works in Westminster, said: “As I came out of the subway on the Downing Street side of Whitehall there was shouting. You could hear the police yell at the man to ‘get down’.
“Officers with guns quickly herded people down the stairs into the underground station for safety. One man who came back down looked as white as a ghost, he was so shaken.
“There were armed police everywhere and crowds of tourists.”
A builder working on the road nearby said: “I saw the police officers catch him. They caught him and got him on the ground.”
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 [the suspect] was very calm. I think they got him on the ground, then they stood him up against the wall.
“I was in New York on September 11, so I am a bit scared of these things and being in the wrong place at the wrong moment.”
Westminster was already on a heightened state of alert following the terrorist atrocity committed by Masood, also known as Adrian Ajao, on March 22.
Armed police had been more visible in Westminster in the aftermath of the onslaught by Masood, 52, who murdered five people including Pc Keith Palmer just inside the grounds of Parliament.
Masood was shot and killed as he tried to enter parliament buildings.
Following yesterday’s incident, Scotland Yard said in a statement: “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.”
The successful arrest is the first real test for Cressida Dick, the new Metropolitan Police Commissioner, who previously - and notoriously - had been in charge of the operation that led to the death of the innocent Brazilian electrician Jean Charles de Menezes who was mistaken for a terrorist when he was shot on the London Underground in July 2005.
Ms Dick briefed London mayor Sadiq Khan on the arrest.
A spokesman for the mayor said: “Mr Khan is hugely grateful to the police for their swift, decisive and professional actions. He is hugely relieved there have been no casualties.”
The investigation into the suspect will now look at how he may have been radicalised and the motives for the attack. Sources said he had not been in Syria.
Islamic State has called on jihadists to launch attacks on centres of power in the West.
It also comes a day after Assistant Commissioner Mark Rowley, the national lead for counter terrorism, warned doctors and health workers were not doing enough to help police prevent terror attacks by mentally ill people who are targeted by extremist recruiters.
Since the 2011 Prevent Review all NHS bodies must train staff to spot the signs of radicalisation and report people they fear are at risk of becoming terrorists to the security services.
Metropolitan Police said that out of 13 major attacks foiled by Scotland Yard since 2013, ‘a disproportionate number’ of those involved people with mental health issues.