
The terror suspect who was detained with a 'rucksack full of knives' near Parliament square had been tracked on an intricate web of CCTV cameras by police and MI5.

Armed police 'swarmed like bees' on the bearded man just before 2.30pm after his concerned family reportedly alerted security services.

Witnesses said the man, who was wearing a tracksuit and trainers, was crossing Whitehall amid the usual crowds of tourists and political staff when a police car suddenly pulled up and officers confronted him, just five weeks after the Westminster terror attack killed that five people.

But, unbeknown to him, the suspect had been tailed from the moment he got off the Underground at St James's Park - where he had travelled from east London - and headed towards Westminster.

While covert teams trailed him on the ground, a live feed was being sent to control rooms in New Scotland Yard and the MI5's Thames House before they gave the order for them to swoop in.

This is the face of a terror suspect tackled to the floor today in Whitehall, where knives were seen strewn across the floor

Police say the man was detained after a stop and search in an 'ongoing operation'. The suspect is now being questioned

Witnesses said the police appeared from nowhere and got the man on the ground. Armed officers stood guard as he was held

At least three knives were seen laying on the floor after a team of officers swooped and tackled the man to the ground

Pictures of forensics officers show at least three knives, a small flick knife, a large one on the ground and another being held

Aerial photographs show police officers grappling with the suspect in the moments as they handcuff and arrest him

The man is then led around to the other side of a waiting unmarked police car, in which he was driven to a nearby police station for questioning

A large number of armed police then flooded the area as the suspect was pinned to the ground just yards from Downing Street. He was held against a wall of the Treasury before being taken away in a van to a police station.

Security sources say police were tipped off by the man's family, which may have allowed officers the chance to detain him before they had to resort to opening fire.

The suspect has not been named but is 27 years old and thought to be known to police, who kept tabs on him via surveillance cameras on the streets and on the Underground.

It is believed that the suspect had been under round-the-clock surveillance and was tracked by two teams today with armed officers on standby, and that his phone and other means of communication were being monitored.

Eyewitness Ian Grant was walking through the area when armed police rushed passed him and arrested the suspect, who is believed to have a British passport, but was not born here.

The 56-year-old, from Poplar in the east of the capital, said: 'I came out of the tube station and saw a police car go flying by. Then all of a sudden it slowed down and two cops got out.

'Then an armed cop with a rifle came passed 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.

'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. '

The area was sealed off as Scotland Yard's forensics experts scoured the scene, where at least three knives and a rucksack were seen laying on the ground on a central reservation.

Police say the arrest was made following a 'stop and search' which was part on an ongoing operation.

He is being questioned by anti-terror detectives at a south London police station.

Mr Grant added: 'Before he was arrested he was just walk along normally, but he must have known they were after him because he didn't say anything when they stopped him.

'It was uncomfortable seeing that happen, it wasn't scary at first, it all happened so fast. But given what happen a month ago, it makes you really nervous.'

The man was arrested just 350ft (116 yards) from where previous attacker Khalid Masood was shot 36 days ago.

A Scotland Yard 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, 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.

The man was tackled on a traffic island before being held against the wall of the Treasury. The bearded suspect - dressed all in black - appeared to have a bandage on his right hand before he was taken away

Police around the Houses of Parliament are on high alert after a terrorist killed five in a car and knife attack five weeks ago

The arrest was made just 300ft from where 'lone wolf' attacker Khalid Masood unleashed his attack five weeks ago

Dramatic photos taken by witnesses show the moment police arrested the suspect after a 'stop and search' check in Whitehall

A witness tweeted this picture of the suspect being held on the ground on the traffic island surrounded by armed police

A picture taken through a phone box shows the man being held against the wall of the Treasury on Whitehall after the arrest

The 27-year-old man, who was wearing a black tracksuit, was ushered away by a team of police around half an hour later

'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.'

Were the Met and MI5 already shadowing the terror suspect? The terror suspect allegedly carrying at least three knives in his rucksack outside Parliament was arrested as part of an 'ongoing operation', Scotland Yard revealed today. It suggests that counter-terrorism officers and MI5 may have been following the 27-year-old before he was held this afternoon. Security sources have said that the suspect was known to the Met before he was pinned to the ground on Whitehall today. Scotland Yard confirmed that the incident was terror-related within minutes of the arrest. And the fact that he was stopped and searched also suggests that the arrest may have been pre-planned. The terror arrest came five weeks after Westminster was put on a heightened state of alert following a major terror attack just over a month ago. Advertisement

A government spokesman for said Theresa May was not in Downing Street at the time of the incident.

The Prime Minister paid tribute to the security services this afternoon.

She said: 'I think it shows our police and intelligence and security services are on alert, as they always are, looking to keep us safe and secure. We owe them a huge debt of gratitude.

'They are often unseen, unheard, yet the job they do, day in and day out, is really important and we should thank them for it.'

Passer-by Jose Noronha, 47, from Southall, west London, saw the man get arrested by police.

He said: 'I saw the cops arresting him as I walked towards the main road. People were everywhere watching and police cars and officers were everywhere.

'Forensic officers have been looking all over the scene now. It was a very scary situation.'

Paul Morris, 29, who works in Westminster, was walking out of the Underground entrance next to where the man was stopped by police.

He 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. It was really busy.'

Forensics experts were seen working on the traffic island, where a bag laying on the ground next to at least two knives

One of the knives is recovered from the scene by a forensic investigator as police officers combed the area for evidence

A forensic investigator was pictured photographing a backpack and set of knives left sprawled on the floor shortly after a man was arrested

Officers were seen piecing through the backpack - which was a cheap 'London' bag of the kind sold at tourist stalls in the area

A rucksack emblazoned with a Union flag and the word 'London' is recovered from the scene by forensic investigators

Forensics officers combed the scene this afternoon after Scotland Yard revealed the arrest was part of an ongoing operation

Bus passenger Ian Moss said police 'looked like they were tending' to him, adding: 'He was arrested. He could walk so no idea if he was injured.'

Asked if he was struggling while on the ground, Mr Moss said: 'No - not at all. He had police pointing arms at him though.'

A builder working on the road across from the scene told MailOnline the incident was over very quickly.

He said he saw the police officers wrestling the suspect, who he described as black, to the ground. The man did not shout and was calm, the witness said.

Eyewitness Luke William said: 'Armed police in Westminster holding a man with a backpack with what appears to be knives falling out on the floor. Stay safe.'

Journalist Helen Chandler-Wilde, 22, was near Westminster Abbey when the commotion unfolded just a few yards away.

She said: 'There are loads of police, about 10 of them, as well as six police cars, and a forensic officer.

'We ran down from just behind Westminster Abbey to see what was going on, we're about 20 yards from the entrance to the Treasury. The traffic is still flowing, it's just a small bit of the road which is closed.'

An employee at HMRC said the main entrance to their building at 100 Parliament Street has been closed.

An photo taken from the air shows the location of the arrest, near the brightly-coloured bus. The man was detained at the southern end of Whitehall, where it opens onto Parliament Square

The investigation took place within view of tourists and reporters at the south end of Whitehall this afternoon

The dramatic arrest unfolded in one of Britain's best known streets, which was busy with tourists and government workers

A police officer guards the scene this evening. The Prime Minister has paid tribute to police and security services

The latest security incident comes after Westminster was put on a heightened state of alert following a major terror attack just over a month ago.

Security in the area has been noticeably more high profile since Khalid Masood killed four people by ramming them with his car on Westminster Bridge before stabbing to death Pc Keith Palmer just inside the grounds of Parliament on March 22.

Parliamentary authorities said they could not comment on what changes had been made to security measures at the Palace of Westminster since the March attack.

In the aftermath of the assault, authorities have reviewed how to keep the area as safe as possible.

Armed police officers have been more visible patrolling the Palace of Westminster since the attack, which also left Masood dead from gunshot injuries.

I AM WAGING JIHAD IN REVENGE AGAINST THE WEST: MOTIVATION OF WESTMINSTER TERROR KILLER KHALID MASOOD IS REVEALED AS SPIES CRACK HIS WHATSAPP MESSAGE The last message sent by the Westminster terror attacker declared he wanted to wage jihad in revenge at military action in the Middle East, it has emerged. Khalid Masood was found to have been active on messaging service Whatsapp just two minutes before he carried out his fatal attack on March 22. But end-to-end encryption has meant authorities have struggled to find out what he sent before he careered his hired car into pedestrians on the Westminster Bridge before attempting to storm Parliament. However, the Independent has learnt that spies have managed to crack into the message, and revealed his motivation was revenge at western military action in the Middle East. The website reports the person who received the message was extensively questioned, but freed after the police and MI5 concluded that he was not part of a plot and had no prior knowledge of what happened. Masood killed five people and injured 50 during his rampage on Parliament last month. Among those were hero police officer Keith Palmer, who was stabbed outside the House of Commons as he acted as the last line of defence. Security sources have called getting into the message a result of 'human and technical intelligence'. The sources have not disclosed how they went about the task, but have confirmed the technical expertise now exists for any future replications. Masood's phone was recovered after he was shot dead by police in the capital. Advertisement

'We owe a huge debt of gratitude': Theresa May praises 'unseen, unheard' security services as Twitter swells with praise for police 'heroes'

Praise: Theresa May has today paid tribute to the 'hero' police and spies behind the arrest of the Whitehall terror suspect

Theresa May has paid tribute to the 'hero' police and 'unseen and unheard' spies behind the arrest of the Whitehall terror suspect today.

The unnamed 27-year-old appears to have been trailed by Scotland Yard officers and MI5 before he was pinned to the ground for allegedly carrying a 'rucksuck' of knives.

Scotland Yard has confirmed that the arrest was terror-related and part of an 'ongoing operation' and that nobody was injured.

Theresa May and many others have praised the authorities for their intervention just yards from Downing Street.

She said: 'I think it shows our police and intelligence and security services are on alert, as they always are, looking to keep us safe and secure.

'We owe them a huge debt of gratitude. They are often unseen, unheard, yet the job they do, day in and day out, is really important and we should thank them for it'.

The Prime Minister was away from Number 10 on a campaign visit at the time of the incident, which did not disrupt work in Downing Street.

Security in the area has been noticeably more high profile since Khalid Masood killed four people by ramming them with his car on Westminster Bridge before stabbing to death Pc Keith Palmer just inside the grounds of Parliament on March 22.

In the aftermath of the assault, authorities have reviewed how to keep the area as safe as possible.

Armed police officers have been more visible patrolling the Palace of Westminster since the attack, which also left Masood dead from gunshot injuries.

Extra concrete barriers have also been installed to form a so-called ring of steel.