Eyewitnesses described scenes of terror as the attacker was shot several times as he approached a second officer within yards of the Houses of Parliament.

A woman assists an injured person on Westminster Bridge. Photo: Reuters/Toby Melville

Paramedics fought to save his life and that of his victim on the floor of the cobbled courtyard in front of Parliament with Foreign Office minister Tobias Ellwood among those who rushed to help.

READ: UK MPs describe chaotic scenes at Westminster

The Port of London Authority said that a seriously injured woman had been pulled alive from the river Thames.

A Reuters photographer saw at least a dozen people injured on Westminster Bridge and photographs showed people lying on the ground, bleeding heavily.

Colleen Anderson, a junior doctor at St Thomas' hospital, said a woman was killed as the attacker drove a grey Hyundai i40 across Westminster Bridge before crashing it into railings then running through the gates of the Palace of Westminster.

She also said she treated a police officer in his 30s with a head injury who had been take to King's College hospital.

She said: "I confirmed one fatality. A woman. She was under the wheel of a bus.

"She died. Confirmed her death at the scene."

At least one of the people being treated in the palace courtyard was wheeled away on a stretcher with their face covered.

The attack left a trail of destruction as paramedics tended to victims on the bridge and at the gate.

Prime Minister Theresa May was seen being ushered into a silver Jaguar in the grounds of the palace as what sounded like gunfire rang out at around 2.45pm.

A Downing Street source confirmed she was "OK".

Knife-wielding assailant

Eyewitness Rick Longley described the attack.

"We were just walking up to the station and there was a loud bang and a guy, someone, crashed a car and took some pedestrians out.

"They were just laying there and then the whole crowd just surged around the corner by the gates just opposite Big Ben.

"A guy came past my right shoulder with a big knife and just started plunging it into the policeman.

"I have never seen anything like that. I just can't believe what I just saw."

Photo: Reuters/Toby Melville

Armed police respond outside parliament in London. Photo: Reuters/Stefan Wermuth

Mayor of London Sadiq Khan said: "There has been a serious incident near to Parliament Square this afternoon which is being treated as a terrorist attack until the police know otherwise.

"I have spoken to the Acting Commissioner. The Metropolitan Police Service is dealing with the incident and an urgent investigation is under way. My thoughts are with those affected and their families.

"I would like to express my thanks to the police and emergency services who work so hard to keep us safe and show tremendous bravery in exceptionally difficult circumstances.

"For the latest information please visit news.met.police.uk."

An air ambulance landed in Parliament Square and a regular ambulance came in through the front gates as medics rushed to help.

Armed police cleared the area around the incident and Parliament Square and surrounding streets were closed to traffic while Parliament went into lockdown.

As the sitting in the House of Commons was suspended, Commons Leader David Lidington told MPs: "What I am able to say to the House is there has been a serious incident within the estate.

"It seems that a police officer has been stabbed, that the alleged assailant was shot by armed police."

Witnesses described seeing a heavy-set man carrying a knife and running towards the Houses of Parliament.

Jayne Wilkinson said: "We were taking photos of Big Ben and we saw all the people running towards us, and then there was an Asian guy in about his 40s carrying a knife about seven or eight inches long.

"And then there were three shots fired, and then we crossed the road and looked over. The man was on the floor with blood.

"He had a lightweight jacket on, dark trousers and a shirt.

"He was running through those gates, towards Parliament, and the police were chasing him."

Her partner David Turner added: "There was a stampede of people running out.

"You saw the people and you thought 'what the hell is going on'."

A witness, who did not want to be named, saw Mrs May and her undercover police detail near the area of Parliament where ministerial cars are parked.

He said there were undercover officers in a 4x4 vehicle in the car park.

"We just heard gunshots inside Parliament," he said.

"It looked like they are taking her away."

Another witness, from South Africa and living in Ireland, said she and two friends went towards the palace gates and saw a man lying on the floor inside.

She said: "I saw a body, they were working on the floor. They were just inside the gates.

"He had one gunshot wound to the right side of his chest. I don't know if he was dead or alive, but people were working on him.

"I saw that needed medical attention and I asked if I could help because I am a doctor, but they said they were fine.

"Then the police moved us back."

Daily Mail political editor Jason Groves witnessed the incident from his office window.

He said: "I heard a bang coming from Westminster Tube station, looked out the window, there's a commotion, armed cops were running, the crowd of people outside were running as if for their lives.

"A guy comes through the vehicle entrance wielding something, towards a copper, the copper tumbles.

"The man's running towards Westminster Hall, and then you've got a plain clothes copper comes running out.

"Not from the gate, but from the inside, shoots him once from maybe 10 or 15 yards away with a handgun, and then gets closer to him and shoots him again from over him and he doesn't get up."

Mrs May's official spokesman said: "The Prime Minister was brought back to Number 10 from Parliament. She is currently monitoring the situation."

US President Donald Trump told reporters at the White House he had been briefed on events in London but gave no details.

In Edinburgh, the Scottish parliament suspended a planned debate and vote on independence as news of events in London came in.

Britain is on its second-highest alert level of "severe" meaning an attack by militants is considered highly likely.

In May 2013, two British Islamists stabbed to death soldier Lee Rigby on a street in southeast London.

In July 2005, four British Islamists killed 52 commuters and themselves in suicide bombings on the British capital's transport system in what was London's worst peacetime attack.