What happened?
At around 14:40 GMT on Wednesday, a car drove into dozens of people on Westminster Bridge in central London.
The car then crashed into railings outside the Houses of Parliament.
The attacker, armed with a knife, ran into the parliamentary estate where he was confronted by the police. One unarmed officer was stabbed and killed.
The attacker was then shot dead by armed officers.
In total, four people were killed including the attacker, while 29 others needed hospital treatment.What was the scene like?
Witnesses have described pandemonium and panic.
One bystander, Richard Tice, said he was coming out of Westminster tube station at about 2:45pm 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?
The police officer has been named as PC Keith Palmer, 48, of the Parliamentary and Diplomatic Protection Command. He had 15 years' service and was a husband and a father.
Another victim has been named as Aysha Frade, a 43-year-old schoolteacher. The mother-of-two was killed after being hit by the attacker's car before it reached parliament.
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.
A woman was given urgent medical treatment after falling into the River Thames as the attacker's car drove onto the pavement.
Police said a "range of nationalities" were among the injured.
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.
Five South Korean tourists were injured - one of them seriously - in a stampede as people fled the area after the attack.
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 said 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 5pm, 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.
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.
