Five people have died and 40 people have been injured in a terror attack near the Houses of Parliament. Here is what we know so far.
At 14.40 GMT a single attacker drove a car over Westminster Bridge, near the Houses of Parliament in central London, killing at least two pedestrians and injuring many more.
The car then crashed into railings outside the Houses of Parliament.
The attacker, armed with a knife, ran to Parliament where he was confronted by the police. One officer – who was not armed – was stabbed and killed.
The attacker was shot dead by armed officers.
What was the scene like?
Witnesses have described pandemonium and panic.
One, Richard Tice, said he was coming out of Westminster tube station at about 14:45 and was ushered by police onto Westminster Bridge.
He saw people lying on the bridge being tended to. He was told that a car had mounted the pavement and driven the whole way, from south to north, across the bridge, knocking people over.
"I counted eight people the length of the bridge, from south to north – at least eight."
Who were the victims?
So far, only the police officer has been named. He was PC Keith Palmer, 48, of the Parliamentary and Diplomatic Protection Command. He had 15 years' service and was a husband and a father.
The injured included three police officers who were walking across the bridge on their way back from a commendation ceremony. Two are described as being in a serious condition.
Police said a "range of nationalities" were among the injured.
Five South Korean tourists were injured, one of them seriously, a tour agency official told the Yonhap news agency.
A group of French schoolchildren were on the bridge and three were injured in the incident.
Four university students from Edge Hill University, in Lancashire, were also hurt. Two were sent to hospital as "walking wounded", whilst two others had minor injuries.
London Ambulance Service said it had treated 12 people with serious injuries, who were all taken to hospital. They also treated eight people with less serious injuries at the scene.
Kings College Hospital says eight patients were being treated there – six male, and two female. Two were described as critical and two stable
St Thomas' Hospital said two patients had been admitted – both were stable.
One patient was admitted to the Royal London Hospital, but no details were given.
Just after 17:00 GMT, a woman was pulled alive from the River Thames, but with injuries.
Who was the attacker? Was there only one?
The prime minister said there was a "single attacker".
He has not yet been named. Police say they think they know who he is, and are "working to look at associates". Acting deputy commissioner Mark Rowley, the Met's top anti-terror officer, urged restraint from "proactive investigative journalists" in working out who he was.
Mr Rowley said the working assumption was that the attacker was "inspired by international terrorism" and "Islamist-related terrorism", but would not comment on his nationality or any other details.
He said police were focusing on the suspect's "motivation, preparation and associates".
What security has been put in place?
Parliament was suspended and politicians, journalists and visitors to the buildings were locked in for about five hours.
Hundreds of people were evacuated from Parliament to Westminster Abbey for safety.
Both the House of Commons and the House of Lords will sit on Thursday at their normal times.
Police said there would be more unarmed and armed police officers on the streets in following days, both in London and across the country.
The UK's threat level has been set at "severe" – meaning an attack is highly likely – for some time and this would not change, the prime minister said.
What about travel in the area?
Westminster underground station has been closed and is open for interchange only.
Police are asking people to avoid the following areas: Parliament Square; Whitehall; Westminster Bridge; Lambeth Bridge; Victoria Street up to the junction with Broadway and the Victoria Embankment up to Embankment tube.