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A suspected terrorist with a 'backpack of knives' was arrested on suspicion of preparing an act of terrorism just yards from the scene of last month's Westminster attack.

Eye-witnesses reported seeing a struggle between the 27-year-old man and police as armed officers descended on the area.

At least 10 police officers and six patrol cars swarmed on Whitehall shortly before 2.30pm as the area around Parliament Street was placed on lockdown.

The man was seen smirking as he was led away by armed police with a bandage wrapped around his hand.

A Western security source told Reuters that the man, from London, was known to counter-terrorism officers and MI5 before his arrest.

Three knives and a rucksack were seen on the ground following the arrest.

(Image: AFP) (Image: PA) (Image: REUTERS) (Image: REUTERS)

The man was arrested on suspicion of the commission, preparation and instigation of acts of terrorism and is being held at a south London police station.

Counter-terror police are now investigating.

Scotland Yard say no-one was injured. The main gates to the Houses of Parliament were shut and armed officers were standing guard.

The scene is just yards from last month's terror attack where Khalid Masood mowed down pedestrians on Westminster Bridge before stabbing PC Keith Palmer to death.

Masood, a British-born convert to Islam, was then shot dead.

(Image: PA) (Image: PA)

The bearded suspect, who was dressed in dark clothing, 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.

Images from the scene showed three knives and what appeared to be a mobile on the ground close to a brown rucksack.

Forensics officers were later seen examining and removing a number of knives from the scene.

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Witness Chris Kacyk, 40, was walking down Whitehall when he saw the suspect being "pounced on" by police.

He said: "I was waiting to cross the road when I saw this guy on the other side being surrounded by a bunch of guys.

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"At first I thought he was being mugged but then he reached into his bag and someone shouted 'armed police' and they pounced on him.

"He seemed very calm and collected, not worried at all. He just looked like anyone you might see in a London street. He was dressed all in black.

"He didn't say anything, even when they were shouting and at him and reading his rights.

"He was cornered. They shoved him up against the wall and were patting him down."

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Fellow eye-witness Ian Grant, 56, from Poplar, east London, added: "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.

"I thought it was just a drill at first. Then he shouted 'Armed police, stop' and they got this guy down in 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.

"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."

Hospitality manager Wojtek Kaczmarkiewicz, was working in the Treasury Building when it was suddenly put on lockdown.

The 34-year-old said: "We were on lockdown for around two hours, it's getting really scary coming to work here to be honest.

"First we knew something was going on was when security locked the door and we had to stay inside.

"Then an email was sent round saying it was over two hours later."

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Prime Minister Theresa May said the handling of the incident near her residence shows that police and intelligence services are always on alert.

She told Sky News: "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 was not at Downing Street at the time because she was campaigning in northern England ahead of the June 8 election.

A Scotland Yard spokeswoman said the man had been detained under the Terrorism Act as part of an 'ongoing operation' and there is no immediate known threat following the arrest.

(Image: SWNS) (Image: SWNS)

She 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 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.

(Image: ITV News) (Image: BBC) (Image: ITV News)

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 latest security incident comes after Westminster was put on a heightened state of alert following the terror attack on March 22.

In the aftermath 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.