Terror returned to the streets of London on Wednesday when a black-clad assailant, armed with two long knives, drove his car through a crowd of people on Westminster Bridge before stabbing a police officer to death on the grounds of Parliament.

Five people died in all, including the attacker himself, in the worst terrorist incident to hit the UK since the 7 July 2005 bombings.

Police said the attacker, who is believed to have acted alone, mounted the pavement of the bridge and mowed down more than a dozen tourists, local workers and police officers before crashing into the Parliament railings.

As people ran for their lives, he entered the gates by Westminster Hall and repeatedly stabbed a male police officer. He was shot three times by the officer's colleagues, and later pronounced dead in hospital.

Police said two people were killed as the car drove down Westminster Bridge. Twenty others were injured in all, including two people who were standing by the railings where the attacker's black Hyundai came to a stop.

Metropolitan Police acting deputy commissioner Mark Rowley said the attack had been declared a terrorist incident, and Theresa May was due to chair a meeting of the Government's emergency Cobra committee later today.

A major counter-terror operation had been launched across the capital, Mr Rowley said, though police believe there was 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.

A man believed to be a suspect in the attack is treated by paramedics (PA)

An armed officer stands with his foot on a knife, as medics provide assistance (PA)

Paramedics treat an injured person on the pavement outside the Houses of Parliament (PA wire)

Tobias Ellwood MP, a foreign minister, was one of the first to arrive on the scene and provided first aid to the police officer, who has not been named while the force offers support to his family.

The minister was pictured, his face bloodied, as he gave up efforts to provide CPR and a sheet was placed over the officer’s body. Later, he confirmed to the BBC that 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.

Images posted to Twitter appear to show the car involved in the attack (Twitter)

The car is understood to have mounted the pavement on Westminster Bridge before ramming the railings outside Parliament

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".

In a statement outside No 10 on Wednesday evening, the Prime Minister praised police and emergency responders who ran towards the danger "even as they encouraged others to move the other way".

She said: "Our thoughts and prayers go out to all who have been affected. To the victims themselves, and their family and friends who waved their loved ones off, but will not now be welcoming them home.

Amber Rudd, the Home Secretary, thanked the emergency services and praised "their bravery, their courage and their professionalism".

"The government's top priority is the security of its people and I urge everyone to remain calm but to be vigilant and if they see anything they are concerned about they should report it to the police," she said.

"We have the best police, the best security services in the world and we must make sure that we let them get on with doing their job.

"The British people will be united in working together to defeat those who would harm our shared values. Values of democracy, tolerance and the rule of law. Values symbolised by the Houses of Parliament. Values that will never be destroyed."

Speaking to reporters outside St Thomas’s Hospital, which is situated immediately at the southern end of Westminster Bridge, junior doctor Colleen Anderson confirmed the first fatality of the attack, a female pedestrian who had been crossing the Thames at the time.

"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 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 lying at the entrance to the bridge. There were 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. "It was so loud,” he said. “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.

An air ambulance arrived and landed in Parliament Square within 10 minutes of the start of the incident (PA wire)

Within half an hour of the incident, Parliament was in total 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 some thought the car smashing into railings sounded like an explosion, and seconds later gunshots were heard.

Chefs, maintenance men, reporters massed at the windows. One filmed as the Prime Minister's car was 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.

Westminster attacks: Footage shows victims being treated outside Parliament

Those were the first actions of a well-rehearsed counterterror plan kicking into motion. In his statement, the assistant deputy commissioner said the Met responded “in line with our plans for a marauding terrorist attack”.

“We are reaching out and engaging with all communities across London to help reassure them,” Mr Rowley said. “Our strength as a city is our ability to stand together at such terrible times.

“If anyone sees anything suspicious or that causes them concern please do contact us - don't hesitate.

"My thoughts are with all those who have been affected by today's attack - and as a service we have lost one of our own as he acted to protect the public and his colleagues. This is a day that we had planned for - that we all hoped would never happen - but sadly it is now a reality.”

MPs wait in the House of Commons as information drifts through via their phones and the Deputy Speaker, David Lidington (BBC News)

MPs remained on lockdown for hours with few updates, keeping spirits high with jokes about how food and drink were – for once – allowed to be consumed within the Commons chamber. They started to file out of the Commons at about 7.30pm, five hours after the incident began.

Parliamentary authorities announced that both Houses would sit at their normal times on Thursday, in a united show of determination not to let the attack disrupt democratic proceedings.

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."

In pictures: Westminster attack Show all 9 1 /9 In pictures: Westminster attack In pictures: Westminster attack An air ambulance lands after gunfire sounds were heard close to the Palace of Westminster in London PA wire In pictures: Westminster attack MPs wait until the situation is under control in Westminster. 'The alleged assailant was shot by armed police,' David Lidington, leader of the House of Commons, told the house. BBC News In pictures: Westminster attack Crowds gather in Westminster after shooting incident, which police are treating as terror attack BBC News In pictures: Westminster attack Police were also called to an incident on Westminster Bridge nearby AP In pictures: Westminster attack Early reports indicate the car, which mounted the pavement on Westminster Bridge and mowed into around a dozen people, was the same vehicle which then rammed into the railings of the Palace of Westminster, just around the corner Reuters In pictures: Westminster attack Security sources described the suspected assailant as a middle-aged Asian man, who is understood to have left the car before attacking a police officer with a seven-to-eight inch knife PA wire In pictures: Westminster attack Police have asked people to avoid the immediate area to allow emergency services to deal with the ongoing incident AP In pictures: Westminster attack One woman has died and a number of others, including the police officer, have been hurt, according to a junior doctor at St Thomas' Hospital Reuters In pictures: Westminster attack At least three gun shots were heard by those inside Westminster, and proceedings in the House of Commons have been suspended AP

Reactions to the attack have come from governments and leaders around the world.

Belgian Prime Minister Charles Michel, who earlier in the day had taken part in commemorations to mark the first anniversary of a terror attack in Brussels, said: "Our condolences are with those who mourn and all who are affected in London. Belgium stands with UK in fight against terror."

The White House said President Donald Trump had spoken to Ms May by phone, and the President praised the response of security forces and emergency services, pledging "the full cooperation and support" of the US government "in responding to the attack and bringing those responsible to justice".

Three French students were identified as being among those injured by the attacker's car on Westminster Bridge. President Francois Hollande was quoted by the French embassy as saying: "Terrorism concerns us all and France know how the British people are suffering today."

Dutch Prime Minister Mark Rutte said: "Horrible images from London. The very heart of the city has been struck. ''Our thoughts are with the British people."

And the Canadian Prime Minister Justin Trudeau said: "Our thoughts are with the victims of today's attack in London and their families. Canadians remain united with the people of the UK."

On social media, people offered their condolences and support to the city under the hashtag “PrayForLondon”, and in a live TV statement on Wednesday night, Mayor Sadiq Khan tried to reassure both the British public and “our visitors” coming to the UK from all over the world.

He expressed his “gratitude, on behalf of all Londoners, to the police and emergency services for their bravery”.