Four people have been killed and around 40 others injured in a terrorist attack in Westminster, Scotland Yard has confirmed.
The attack began when a car ran down pedestrians on Westminster Bridge. A police officer was subsequently stabbed and died as a result of the injuries sustained. He was named as 48-year-old Keith Palmer.
Police said three members of the public were also killed during the attack, which happened around 2.40pm on Wednesday afternoon.
The attacker died after being shot by police outside parliament.
Theresa May condemned the attack as "sick and depraved" while London mayor Sadiq Khan offered his thanks to the police and emergency services, and said they had showed "tremendous bravery".
On Wednesday evening anti-terrorism officer Mark Rowley said police were "satisfied at this stage it looks like there was only one attacker", but added they could not yet be certain.
At least 40 others were injured, the police said. Some of the injured were police officers.
Witnesses reported seeing a car hit several people on Westminster Bridge ahead of a knife attack, while a woman was also rescued from the Thames.
"The thoughts of the Prime Minister and the government are with those killed and injured in this appalling incident, and with their families," a Downing Street spokesperson said on Wednesday afternoon.
"The PM is being kept updated and will shortly chair COBR [the Cabinet Office Briefing Room, also known as Cobra]."
The City of London Police said: "People can expect to see a heightened police presence in the City of London as a precautionary measure. We are liaising with the Metropolitan Police to co-ordinate our response. As always, if anyone sees anything then we of course encourage them to get in touch."
French media reported that three French teenage students from the Saint-Joseph school in Concarneau were injured in the attack.
The incident came as Belgium marked the first anniversary of the Brussels terror attacks.
"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," said home secretary Amber Rudd.
"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."
Read more: What the emergency services know so far
Following the attack, Leader of the House David Lidington MP said the Commons would "remain in lockdown until we receive advice that it is safe to go back to normal procedures". Some people in the Palace of Westminster were later evacuated, with many MPs, peers and bystanders sheltering in Westminster Abbey, although parts of the building remained on lockdown into Wednesday night. Nonetheless, parliamentary authorities say both the Commons and Lords will sit as usual on Thursday.
Anna Soubry MP said colleagues were led from the Commons library "at gunpoint" by counter terrorist armed police officers.
Colleagues taken out of the library at gun point by counter terrorist armed police officers.— Anna Soubry MP (@Anna_Soubry) March 22, 2017
Read more: David Lidington's statement to MPs in full
Reuters reported that multiple people were injured by a car on Westminster Bridge. Property developer Richard Tice was in Westminster at the time, and told Sky News he "counted eight to 10 prostrate figures on the ground... from north to south along Westminster Bridge".
Grant Shapps MP tweeted that he was walking through the Commons cloisters to vote, when he heard four gunshots. Police had then MPs hit the ground and crawl to cover.
Walking thru Commons cloisters to vote, heard 4 gunshots. Police had MPs hit ground & crawl to cover.— Grant Shapps MP (@grantshapps) March 22, 2017
In the aftermath of the attack, the Metropolitan police asked 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. Nearby tube stations were closed.
"This is to allow the emergency services to deal with the ongoing incident," the Met said in a statement.
Police were called at approximately 14:40 hours to reports of an incident in Westminster Bridge, SW1.
"Officers – including firearms officers – remain on the scene and we are treating this as a terrorist incident until we know otherwise," the police said at the time.
The police directed people to follow its Twitter account for details.
We were called at approx 2:40pm to reports of an incident at #Westminster Bridge. Being treated as a firearms incident - police on scene— Metropolitan Police (@metpoliceuk) March 22, 2017
We are aware of reports of an incident at #Westminster. We will put more information out as soon as we can— Metropolitan Police (@metpoliceuk) March 22, 2017
Officers – including firearms officers - are on scene and dealing with the incident in #Westminster— Metropolitan Police (@metpoliceuk) March 22, 2017