Four people are confirmed to have died in a car and knife attack on Westminster in central London, including a police officer who was protecting Parliament and the suspected assailant, who was shot dead by armed police.
The attacker, dressed in black and armed with two large knives, drove a car over Westminster Bridge, ramming into members of the public before crashing his car into the rails outside the Houses of Parliament.
Police said the attacker, who is believed to have acted alone, then entered the gates of New Palace Yard and repeatedly stabbed a male police officer. He was shot by the officer's colleagues.
Two people were killed when the attacker's car mounted the bridge's pavement. Twenty others were injured in all, including two people who were standing by the railings where the attacker's black Hyundai i40 came to a stop.
Metropolitan Police assistant commissioner Mark Rowley said the attack had been declared a terrorist incident, and Theresa May is due to chair a meeting of the Government's emergency Cobra committee later today.
Mr Rowley said a major counter-terror operation was underway, though police currently believe there is only one attacker. "Looking forward, people of London will see extra police officers on our streets, and we could call on the support of military," he said.
The attack began at around 2.40pm and proceedings in the House of Commons were immediately suspended. Deputy speaker David Lidington announced to the House that a police officer had been stabbed and the "alleged assailant shot by armed police".
Security sources have described the suspected assailant as a middle-aged Asian man, who attacked the officer on foot with a seven-to-eight inch knife.
Images have emerged of a man dressed in black, believed to be the suspect, being treated on a stretcher within Parliament grounds. A knife could be seen lying on the ground nearby.
Tobias Ellwood, a foreign minister, was one of the first to arrive after the police officer was stabbed and provided first aid.
Earlier, he told the BBC the officer had died at the scene.
Witnesses, including members of The Independent's lobby staff, said the police officer fell to the ground clutching his arm or shoulder after he was stabbed, and was seen wounded but moving in the moments afterwards.
A man, believed to be the assailant, then tried to run towards the exit of New Palace Yard, underneath Big Ben.
Police were seen shouting at him, presumably to stop. Shots were then heard.
Dozens of police, many armed, ran around the gate of parliament in the moments after the shots were fired.
Witness Rick Longley said he saw the car crash into the Parliament railings, as well as the subsequent stabbing.
"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," he said.
"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."
Theresa May was still on the parliamentary estate following Prime Minister's Questions when the attacker struck.
Witnesses saw the Prime Minister being led to her car, a silver Jaguar, with the vehicle about to leave when officers then prevented it from driving into New Palace Yard where the incident occurred.
A Downing Street source confirmed Ms May was "OK".
Speaking to reporters outside the central London hospital, junior doctor Colleen Anderson from St Thomas' Hospital said a female pedestrian had died on Westminster Bridge.
"She was under the wheel of a bus. She died, [medics] confirmed her death at the scene."
She also said she treated a police officer in his 30s with a head injury who had been taken to King's College Hospital.
In his statement to the press, Mr Rowley said three officers were among those injured on the bridge.
Ms Anderson said the injuries to those walking on the bridge at the time ranged from minor to "catastrophic".
One woman was pulled from the River Thames after falling or jumping from Westminster Bridge during the incident.
A spokesman for the Port of London Authority said: "A female member of the public was recovered from the water near Westminster Bridge. She is alive but undergoing urgent medical treatment on a nearby pier. We believe she fell from the bridge."
He said the river has been closed from Vauxhall to Embankment "as part of the security response".
One witness told Sky News: “It was fairly busy and I was just walking across the bridge when suddenly a bus stopped right in front of me and everybody started screaming.
"I saw a trainer in the road and I thought somebody must have been hit by a car, but then I saw a body on one side of the road.
“Then there was another body further up - and then when I looked over the side of the bridge there was another body in the water."
Van driver Mitchell Spree, 27, told The Independent he was driving along Embankment when he saw people being evacuated from nearby buildings.
He said: "Then we pulled on to the bridge. A lady was laying at the entrance to the bridge. There was about five more people.
"She was crying. She was speaking to the paramedic. I don't know what the others were like. The police asked us to leave our van. It's still on the bridge.
"It's shocking."
A second witness, Tawhid Tanim, told The Independent he heard three shots - "bang, bang, bang" - some 10 or 15 metres from the Cafe Nero coffee bar where he was waiting for friends.
He said: "It was so loud. People were running like crazy."
He saw the aftermath of the car striking railings outside Parliament, he said. Whether someone, or something, was underneath, he could not be sure.
"I couldn't see it properly. I started running."
Police officers told the crowds to "just keep running," he said.
Within half an hour of the incident, Parliament was in lockdown. The chamber cleared of MPs, the restaurants of diners. Parliamentary passes no longer opened any door, any gate.
Westminster's press offices occupy the top floor of the palace along from Big Ben and round to Westminster Hall, right over the courtyard where a car smashed into a gate, tourists and pedestrians screamed and, seconds later gunshots were heard.
Chefs, maintenance men, reporters massed at the windows. One had a video of the Prime Minister's car, understood to be Theresa May hurriedly driven away.
Half an hour after the incident, along the Committee Corridor, where MPs meet for meetings all week, another wave of shouting could be heard. Through the windows, anti-terror police could be seen, in their air force blue uniforms, barking instructions to one another, apparently securing the building.
Labour MP Mary Creagh told reporters: "They are clearing it floor by floor.
"It was very frightening, to see people running towards you, to hear that shots had been heard."
"My thoughts are really with those people" who were victims of the attack, she said.
A joint statement from Commons Speaker John Bercow and Lord Fowler, the Lord Speaker, said: "On behalf of Members of both Houses of Parliament, we wish to offer our thoughts to all those affected and their families.
"We would also like to express our gratitude to the police and all emergency services."
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