The Metropolitan Police confirmed five people had been killed, including the stabbed police officer and the attacker himself, in a chaotic incident which saw the Parliament on lockdown for hours and the heart of the British capital brought to a standstill.

About 40 other people are being treated for injuries, some of which are described as "catastrophic", after the multi-pronged attack, in which an unidentified man used both a vehicle and knife as deadly weapons. 

The chaos began when a vehicle being driven over the Westminster Bridge mowed down a number of pedestrians, including several police officers and children, causing serious injuries and leading to at least one victim falling or jumping from the bridge into the River Thames. 

The driver then crashed the car into a fence outside the Houses of Parliament, before getting out and trying to run inside. The assailant, armed with a knife, stabbed one police officer who was guarding the building, before he was shot dead by other officers.

Despite efforts by a Conservative MP and others to resuscitate the injured police officer, he passed away.

He has since been named as Keith Palmer, a 48-year-old husband and father with 15 years policing experience. Police said Mr Palmer was not armed at the time of the attack. 

The assailant has not been identified. London's assistant police commissioner Mark Rowley said police think they know the identity of the man but would not reveal details. He said Islamic extremism is suspected in the attack.

British Prime Minister Theresa May condemned the "sick and depraved" terror attack and conveyed her condolences to the families of the victims.

"For those of us who were in Parliament at the time of this attack, these events provide a particular reminder of the exceptional bravery of our police and security services who risk their lives to keep us safe," she said on Wednesday evening.

The attack comes on the one year anniversary of a terror attack in Brussels, in which suicide bombers killed more than 30 people at an airport and metro station.

Australian Foreign Minister Julie Bishop said she knew of no Australians involved in the attack.

EARLIER: Four people are dead and the British Parliament remains in lockdown after a terrorist attack in central London, in which a car mowed down a number of pedestrians and a police officer was stabbed outside the parliament.

The Metropolitan Police confirmed four people had been killed in the chaotic incident, including the stabbed police officer and the attacker. At least 20 other people are still being treated for injuries, some of which are described as "catastrophic", after what appears to be a multi-pronged attack in the British capital using both a car and knife.

The chaos began when a car driving over the Westminster Bridge ploughed into a number of pedestrians, including several police officers, causing serious injuries and leading to at least one victim falling or jumping from the bridge into the River Thames.

The car then crashed outside parliament, where a man armed with a knife got out and tried to enter parliament, according to police. The assailant stabbed a police officer before being shot by police. Despite efforts by a Conservative MP and others to resuscitate the injured police officer, the officer died.

The attack comes on the one year anniversary of a terror attack in Brussels, in which suicide bombers killed more than 30 people at an airport and metro station.

Pedestrians mowed down

Just after 2.30pm, local time, London's emergency services were contacted about a incident on the Westminster Bridge in the heart of the capital, where a vehicle had struck a number of people.

Former Polish foreign minister Radoslaw Sikorski told the BBC he was in a taxi driving along the bridge when he heard what he thought was a "collision".

"And then I looked through the window of the taxi and I saw someone down, obviously in great distress," he said. "And then I saw a second person down. I started filming, and then I saw three more people down, one of them bleeding profusely.

"What I saw was an incident involving at least five people seriously injured."

Mr Sikorski said he started filming "seconds after the car passed", describing it as "mowing down" pedestrians.

"People rushed forward to help the victims," he said. "I saw five people down."

In the video Mr Sikosrki posted to social media, shocked witnesses rush to help a man lying unmoving on the roadway. One woman shouts "oh my God" as she runs to the man's side, before running down the road to get help. Another man clutches his leg as two passersby kneel beside him.

Eyewitness Steve Voake told the BBC he was walking across the bridge when he saw two bodies lying on the ground - and another in the water.

Assailant shot by police

London's assistant police commissioner Mark Rowley said police believed the vehicle moved from Westminster Bridge and crashed near the parliament, which sits right beside the bridge.

"At least one man, armed with a knife, continued the attack and tried to enter parliament," he told reporters.

Witnesses reported seeing the assailant rush towards the building and stab an officer.

Tom McTague, the chief UK political correspondent for Politico, said on Twitter he saw "attacker runs through gates, stabs policeman, runs forward & then police shoot at least twice, he falls."

Jayne Wilkinson, was near the statue of Winston Churchill in Parliament Square with her partner, David Turner, said they had seen a middle-aged man holding a knife. He ignored warnings from the police, running though the gates into the compound of Parliament, she said.

"They were shouting to warn him," Wilkinson said.

Soon after, she and her partner heard three gunshots and saw the man on the ground.

Dennis Burns, who was just entering Parliament for a meeting when the security alert happened, told the Press Association: "As I walked in I heard a security guard get a radio message saying 'a policeman has been stabbed'.

"Then I walked in as police officers and security start rushing out of the front doors on to the street.

"When I got inside I was wondering what the hell was going on and I saw dozens of panicked people running down the street. The first stream was around 30 people and the second stream was 70 people.

"It looked like they were running for their lives."

British Tory MP Tobias Ellwood was photographed trying to resuscitate the stabbed officer outside parliament.

Assistant commissioner Rowley confirmed the police officer had died in the attack and that his family had been notified.

"This is a day we planned for but hoped would never happen," he said. "Sadly it's now a reality. We will continue to do all we can to protect the people of London."

Mr Rowley said he believed the slain assailant was the sole attacker in the incident. He would not say whether the man was known to police.

'Catastrophic injuries'

The two-stage attack left at least 20 people injured, including a woman who was pulled alive from the Thames and several schoolchildren visiting from France.

The first confirmed fatality was a woman who died on the road following the vehicle attack.

"She was under the wheel of a bus... She died, confirmed her death at the scene," said Dr Colleen Anderson from St Thomas' Hospital.

"There were people across the bridge. There were some with minor injuries, some catastrophic."

London police later confirmed the death toll had risen to four, including the police officer and the attacker, and that there number of injured was at least 20.

US condemns attack

A White House spokesman confirmed US President Donald Trump has spoken to Prime Minister Theresa May about the incident. US Secretary of State Rex Tillerson extended the US' sympathies to the people of Britain.

"We condemn these horrific acts of violence, and whether they were carried out by troubled individuals or by terrorists, the victims know no difference," he said in a statement.

Prime Minister Theresa May was safe after the incident, a spokesman for her office said. He declined to say where Mrs May was when the attack took place.

Parliament locked down

Fairfax reporter Latika Bourke remains inside the British Parliament.

Bourke was with a group of 14 journalists who were receiving a briefing from Lord Boswell, the principal deputy chairman of Lords committees, when a breaking news alert flashed up on their phones, saying there had been a "shooting" outside Parliament.

"One Turkish journalist immediately yelled out the distressing news causing a slight commotion," she says.

"Minutes later we noticed the television on the wall screening the Commons that sitting had been suspended.
Journalists were informed that no-one was allowed to leave the Houses of Parliament."

Martin Vickers, Conservative MP for Cleethorpes, told Fairfax he had been on his way through the Westminster cloisters - metres from the site of the attack when it occurred.

He and a group of colleagues had been heading into the house for a vote, responding to the division bells.

"Two or three shots - well, what we now know to be shots, rang out," Mr Vickers said

"Then security people were pushing us towards the doors and saying 'please get out of the building'."

He and the other MPs left in "a reasonably orderly fashion", he said. In the confusion he hadn't even been sure they were gunshots.

"Everyone was apprehensive ... but there was no real panic at all."

He found himself outside Westminster near the bridge.

"I saw the vehicle lodged against the railings but at that stage the security people were telling everyone to get as far away as possible."

He said he had been impressed by the response of the emergency services.

"Ultimately the democratic process and freedom will be the winner in this," he said. "We will not kowtow here in Britain or any other parts of the democratic world.

"Life will have to go on."

With The New York Times