A SUDANESE man who is known to police is believed to have been behind the terror attack on Westminster.

Salih Khater, 29, veered off the road and hit pedestrians and cyclists at Parliament Square on Tuesday morning.

It came as British counter terrorism police are carrying out searches at three properties in the Midlands area of England for clues as to the motive of the car attack.

Three people were injured when the silver Ford Fiesta driven by the man - thought to be Khater - when he rammed into a group of cyclists and pedestrians before crashing into a security barrier outside the Houses of Parliament.

The UK national remains in police custody in south London and has been arrested on suspicion of preparation of a terrorist act.

He is being held under provisions in the UK’s Terrorism Act. It is understood that he is refusing to cooperate.

One of the victims was treated for minor injuries at the scene, while a man and a woman were taken to with non-life-threatening injuries.

Both were discharged overnight.

The Metropolitan Police Service said the attack appeared to have been deliberate and it is “being treated as a terrorist incident” led by the Met’s Counter Terrorism Command.

It comes as shocking CCTV footage showed the moment the car swerved and hit at least 10 cyclists and pedestrians before smashing into barriers outside Britain’s Houses of Parliament.

The suspect, a man in his late 20s, was arrested by armed police after being pulled from a silver Ford Fiesta hatchback.

“Given that this appears to be a deliberate act, the method, and this being an iconic site, we are treating it as a terrorist incident,” London Assistant Commissioner Neil Basu said.

The CCTV footage showed the car swerving the wrong way down the road before veering into pedestrians at 7.37am local time (4.37pm AEST).

London’s Metropolitan Police Service said that the man, aged in his late 20s, was “arrested on suspicion of terrorist offences”.

Although he had not been formally identified, the man was not believed to be known to police, Mr Basu added.

“There was nobody else in the vehicle, which remains at the scene and is being searched,” police said in a statement. “No weapons have been recovered at this stage.”

US President Donald Trump called on the UK to be tougher on terrorists.

Eyewitnesses said the driver had crashed into the bollards and “there was a loud bang”.

“I saw the cyclists, injured cyclists. I don’t know if he’s hit these people, or if they’ve just dived to escape,” witness Jason Williams told reporters. “It didn’t swerve, there was not another car going behind him. It looked like it was planned.”

“Straight away I thought ‘Oh no, this is another terrorist attack’. So I’ve just started to run and the police were saying ‘get out, get out of the area’,” he said.

“I have seen people on the ground, lying on the road. I don’t know if they have actually been hit by the vehicle or not. I saw at least 10 people lying down. I had to run for my life.”

British Prime Minister Theresa May, who like other MPs is on holiday during parliament’s summer recess, said her thoughts were with the people injured.

“All Londoners, like me, utterly condemn all acts of terrorism on our city,” London’s Mayor Sadiq Khan said on Twitter.

A massive police response has seen the Westminster area closed off.

Several high-value locations exist in Westminster, with many important political premises in the area.

There is a heavy armed police presence outside the building after the car crash.

All entrances to the Houses of Parliament were placed in lockdown. No one was allowed in or out.

Those outside were asked to move back as Parliament Square was closed with dozens of police and emergency vehicles at the scene.

Armed police approached the small silver hatchback as uniformed police moved members of the public back and established a perimeter.

The driver was surrounded by officers and led away from the vehicle in handcuffs.

The arrested man was calmly led away from the scene, while smoke billowed from his car.

British Parliament is in recess, however thousands of government employees continue to work in the Westminster area.

The location is also popular with tourists.

An eyewitness told Sky News he believed the car had “deliberately” driven towards the pedestrians.

Metropolitan Police confirmed in a tweet that at 7.37am local time a car collided with the barriers outside Parliament.

They also confirmed a number of pedestrians had been injured, and the male driver had been detained at the scene.

“At 0737hrs today, a car was in collision with barriers outside the Houses of Parliament. The male driver of the car was detained by officers at the scene,” Scotland Yard said in a statement.

“A number of pedestrians have been injured.

“Officers remain at the scene.

“At this stage, officers do not believe that anybody is in a life-threatening condition,” Metropolitan Police also said in a statement.

“Cordons are in place to assist the investigation.”

Westminster Tube station was closed down and Westminster Square blocked off from the public.

BBC News Political Correspondent Jonathan Blake also tweeted that armed police at the scene told onlookers: “Move back as quick as you can, we’re asking you for a reason.”

The incident evoked memories of recent terror incidents which have hit London, including an attack at nearby Westminster Bridge in March 2017 during which 52-year-old Khalid Masood drove a car onto the footpath, killing five people and injuring more 50 people.

In June 2017, eight people were killed and 48 injured when three terrorists drove a van into pedestrians on London Bridge before leaving the van and stabbing dozens of people in nearby bars at Borough Market.

Protective barriers around Westminster were extended in the wake of the 2017 attacks.