Five people have been killed and about 40 injured in London after a car ploughed into pedestrians and a suspected Islamist-inspired attacker stabbed a policeman close to Britain's parliament.
The dead, in what police called a "marauding terrorist attack," included the assailant and the policeman he stabbed on Wednesday (local time).
The other three victims were among those hit by the car as it sped across Westminster Bridge before crashing into railings just outside parliament about 3pm. The assailant was shot by police just outside the building in central London.
Prime Minister Theresa May condemned Wednesday's attack as "sick and depraved."
"The location of this attack was no accident," she said in a statement outside her 10 Downing Street office late in the evening.
"The terrorist chose to strike at the heart of our capital city, where people of all nationalities, religions and cultures come together to celebrate the values of liberty, democracy and freedom of speech."
Any attempt to defeat those values through violence was "doomed to failure," she said.
The officer killed has been named by the Met Police as Constable Keith Palmer, a 48-year-old husband and father.
Mark Rowley, Britain's most senior counter-terrorism officer, told reporters the attack started when a car was driven over Westminster Bridge, hitting and injuring members of the public and three police officers.
"A car then crashed near to parliament and at least one man, armed with a knife, continued the attack and tried to enter parliament," Rowley said.
He said the police's "fast-paced investigation" was working on the assumption that the attack was "Islamist-related terrorism." Police believed they knew the identity of the attacker but would not provide details at this stage, he said.
It was the deadliest attack in the capital since four British Islamists killed 52 commuters and themselves in suicide bombings on the city's transport system in July 2005, in London's worst peacetime attack.
Reuters reporters inside parliament at the time heard loud bangs and shortly afterwards saw the knifeman and the stabbed policeman lying on the ground in a courtyard within the gates of parliament.
A Reuters photographer saw at least a dozen people injured on the bridge. His photographs showed people lying on the ground, some of them bleeding heavily and one under a bus.
A woman was pulled alive, but with serious injuries, from the Thames, the Port of London Authority said. The circumstances of her fall into the river were unknown.
Several MPs and senior officials were caught up in the chaos. Tobias Ellwood, a junior Foreign Office minister, was pictured attempting to resuscitate a man lying unconscious, reported to be the stabbed policeman.
Three French schoolchildren aged 15 or 16 were among those injured in the attack, French officials said.
Five South Koreans were injured near parliament, it has been confirmed.
Three women and one man in their 50s and 60s suffered injuries including broken bones, the South Korean foreign ministry said. Another woman in her 60s suffered a head injury while falling and was taken to a hospital to undergo surgery.
London Mayor Sadiq Khan said there would be additional police officers on the city streets to keep Londoners and visitors safe.
"We stand together in the face of those who seek to harm us and destroy our way of life. We always have, and we always will. Londoners will never be cowed by terrorism," he said.
Warning: video contains graphic descriptions
CHAOTIC SCENES
The attack took place on the first anniversary of attacks by Islamist militants that killed 32 people in Brussels. Eyewitnesses described chaotic scenes during the attack.
"I just saw a car go out of control and just go into pedestrians on the bridge," said Bernadette Kerrigan, who was on a tour bus on the bridge at the time, in an interview with Sky News.
"As we were going across the bridge, we saw people lying on the floor, they were obviously injured. I saw about 10 people maybe. And then the emergency services started to arrive. Everyone was just running everywhere."
Later, May said parliament would convene as normal on Thursday in a sign that the attack would not disrupt life in the capital.
However, a visit by the Queen to officially open the new headquarters of London's police force, which had been planned for Thursday, was postponed "in light of today's events," Buckingham Palace said.
May has spoken to US President Donald Trump about the attack, the White House said, while US Secretary of State Rex Tillerson condemned what he described as "horrific acts of violence."
Polish former Foreign Minister Radoslaw Sikorski, who saw the attack from a taxi as he crossed Westminster Bridge, said he saw five victims and made a video of the scene
"I heard something that sounded like a small car crash. Then I looked out of the window and saw that there was one person lying on the asphalt. One of the men I saw, his head was bleeding very badly. But the person I filmed - no, that person was not showing any signs of life."
'Horrible to watch'
Journalist Quentin Letts of the Daily Mail newspaper told LBC radio that he had witnessed the stabbing of the policeman and the shooting of the assailant from his office in the parliament building.
"He (the assailant) ran in through the open gates ... He set about one of the policemen with what looked like a stick," Letts said.
"The policeman fell over on the ground and it was quite horrible to watch and then having done that, he disengaged and ran towards the House of Commons entrance used by MPs (members of parliament) and got about 20 yards or so when two plain-clothed guys with guns shot him."
Some people suffered catastrophic injuries in the incident, British news agency the Press Association reported, citing a doctor at St Thomas's Hospital which is located directly across the River Thames from parliament.
Reuters reporters inside parliament saw a large number of armed police, some carrying shields, pouring into the building.
World leaders offer support
German Chancellor Angela Merkel expressed shock at the attack and she said her thoughts were with the British people and everyone in the capital.
Merkel stressed that Germany was on the same side as Britain in terms of fighting terrorism. In December, a failed Tunisian asylum seeker pledging allegiance to Islamic State killed 12 people in Berlin by driving a truck into a crowded festive market.
President Donald Trump on Wednesday offered Britain the full cooperation and support of the United States.
In Edinburgh, the Scottish parliament suspended a planned debate and vote on independence as news of events in London came in.
London attacks
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.
Wednesday's attack was near the location where, in 1979, Airey Neave, a Member of Parliament who was a close aide to Margaret Thatcher, was killed by a car-bomb claimed by the Irish National Liberation Army.
