Five people are now confirmed to have died in today's suspected terror attack at the Palace of Westminster.
An unarmed police officer and the attacker himself are among the dead, while around 40 others were injured.
The attacker, armed with two large knives, mowed down pedestrians with a car on Westminster Bridge, including schoolchildren, then rushed at the gates in front of the Houses of Parliament, stabbing the policeman before being shot dead by other officers.
The fatally wounded policeman was named by Scotland Yard's top anti-terror officer Mark Rowley as 48-year-old husband and father Keith Palmer.
Mr Rowley told reporters outside New Scotland Yard, a short distance from the scene of the attack, that the death toll had been updated to five - including Mr Palmer, the suspect, and three members of the public.
Paying tribute to Mr Palmer, Mr Rowley said: "He was someone who left for work today expecting to return home at the end of his shift - and he had every right to expect that would happen."
Some of those hurt have been described as suffering 'catastrophic' injuries.
It came as the Prime Minister vowed to defeat what she called 'the forces of evil'.
Mrs May, who was in Parliament at the time of the attack, praised the bravery of police officers who killed the attacker as he sought another victim.
She said any attempt to defeat the values that Parliament stood for was "doomed to failure".
Mr Rowley said police believed they knew who the suspect was, but would not go into further details.
Paramedics fought to save Mr Palmer's life, and that of his attacker, on the floor of the cobbled courtyard in front of Parliament, with Foreign Office minister Tobias Ellwood among those who rushed to help.
The police officer was wheeled away on a stretcher with his face covered.
Mr Ellwood, who lost his brother in the Bali bombing, could be seen pumping the officer's chest then standing above him, his hands and face smeared with blood.
Armed officers, some in plain clothes and wearing balaclavas, swarmed around the yard just feet from where MPs had earlier attended Prime Minister's Questions.
The knifeman drove a grey Hyundai i40 across Westminster Bridge before crashing it into railings, then running through the gates of the Palace of Westminster.
His attack left a trail of destruction as paramedics tended to victims on the bridge and at the gate.
One woman hit by the attacker's car before he reached Parliament was confirmed dead by a doctor at St Thomas' Hospital. She said others on the bridge suffered "catastrophic injuries".
Another woman who fell into the Thames was rescued and given urgent medical treatment on a nearby pier.
London Ambulance Service said paramedics had treated at least 10 patients on Westminster Bridge.
A party of French schoolchildren were among those targeted on the bridge, with three injured.
Mobile phone footage showed a woman lying lifeless under a bus on the bridge near where the car crashed to a halt.
Eyewitness Rick Longley said: "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."
Rob Lyon, 34, from Rugby, Warwickshire, was walking along Westminster Bridge with a colleague when he saw the car travelling at high-speed, hitting pedestrians.
He said: "I heard a wheel definitely hit a kerb, quite a loud crunch noise, I looked up and saw a car clearly hitting people as it came towards me.
"A colleague I was with, James, I heard him sort of shout. I instinctively jumped off the pavement. I could see people being hit.
"And then the car just carried on up the bridge and I just looked around and was really in shock."
He said he saw about five people lying on the ground around him.
"(I saw) people that had been hit by a car at speed - it was harrowing," he said.
Steve Voake, 55, was walking across Westminster Bridge towards the South Bank when he saw the aftermath.
He said he saw at least two bodies lying on the road and one in the water.
"I saw a trainer lying in the road and when I looked more closely I saw that there were a couple of bodies the other side of the road," he told the Press Association.
"And when I looked over the side there was another body lying in the water with blood all around it."
An air ambulance landed in Parliament Square and a regular ambulance came in through the front gates as medics rushed to help the police officer.
Armed police cleared the area around the incident, and Parliament Square and surrounding streets were closed to traffic while Parliament went into lockdown.
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."
Frazer Clarke, 25, from Burton-on-Trent, Staffs, one of a group of young boxers who witnessed the attack, said: "We heard a loud bang and screaming and then I noticed some smoke. I thought it was a car crash.
"I looked towards the front gate and people were running, a police officer and a fellow coming to the gate with two knives.
"He was stabbing the police officer with the knives.
"He was wearing black tracksuit bottoms, a black or grey top and what looked like work boots.
"The police officer was stumbling and fell on the floor.
"Another police officer walked toward the guy with the knives and shot him twice."
Donald Trump was among world leaders quick to offer their support.
The White House said the US president spoke to Theresa May in a phone call following the bloody events in London.