Westminster is on lockdown this afternoon after a "terror attack" left a woman dead and at least a dozen people hurt - including some with "catastrophic" injuries - outside the Houses of Parliament.
Gunfire was heard just before 3pm in central London, with a government minister confirming a police officer was stabbed and the assailant shot by armed police.
There are reports of further violent incidents nearby, and police say they have been called to a firearms incident on nearby Westminster Bridge.
Witnesses said a vehicle struck several people on the bridge, and photos showed a car plowed into railings.
Scotland Yard said the attack in Westminster was being treated "as a terrorist incident until we know otherwise".
The Metropolitan Police added in a statement that the incident is ongoing and are urging people to stay away from the area.
The Parliament complex is now on lockdown. Deputy Commons Speaker Lindsay Hoyle told MPs they would be the last parliamentary staff to be evacuated, and would be moved out in groups of 20.
Witness Rick Longley told the Press Association that he saw a man stab a policeman outside Parliament.
"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."
Another eye-witness told The Huffington Post UK that the suspected assailant appeared to attack a police officer who fell to the ground.
Shots were then fired and the assailant fell to the ground as he tried to get out of New Palace Yard, the witness said.
Reporters at Westminster said they could see at least one body covered with a blanket while another person was receiving CPR before being put in an ambulance.
Sky's Tamara Cohen added that the Prime Minister was "bundled out [of Parliament] by [a] plain clothes policeman" when the news broke.
MP trapped in chamber says the Prime Minister was in the division lobby when it happened, bundled out by plain clothes policeman
— Tamara Cohen (@tamcohen) March 22, 2017
A Downing Street source declined to say where May had been during the attack, but said she had not been involved.
A separate eyewitness who was in Parliament at the time told HuffPost UK: "I heard what sounded like an explosion and raised voices outside Parliament, and so I rushed to the window.
"I saw people running past the entrance to the New Palace Yard entrance to Parliament, and then at least one person try to run into the Yard itself. A police officer chased this person and wrestled them to ground.
"Shots were then fired, but I can't remember how many and I didn't see who fired them.
"I ran down the stairs from my office overlooking the Yard to get more information, but the police prevented us from leaving the stairwell.
"When I returned to my office, I could see two people lying on the ground outside Westminster Hall, but neither of these were the person I saw wrestled down by the police officer minutes earlier."
Other political reporters based in the area shared their version of events:
Emergency services trying to revive an individual lying on pavement some way away from entrance to parliament on Parliament Street. pic.twitter.com/bmytpXZ11p
— Jim Waterson (@jimwaterson) March 22, 2017
Shots fired outside Parliament. Loud explosion then shooting. Man lying shot outside gates to Parliament. Gun shots outside, Frightening.
— Christopher Hope (@christopherhope) March 22, 2017
Just seen police shoot a man outside parliament, appeared to be wielding a knife
— Charlie Cooper (@CharlieCooper8) March 22, 2017
The House of Commons was briefly suspended, but after a statement from its Leader David Lidington, it was adjourned.
Lidington told MPs in the chamber: "What I am able to say to the House is that there has been a serious incident within the estate.
"It seems that a police officer has been stabbed that the alleged assailant was shot by armed police. An air ambulance is currently attending the scene to remove the casualties.
He continued: "It would be wrong of me here to go into further details until we have confirmation from police and from the house security authorities about what is going on."
Jeremy Corbyn said afterwards that his "thoughts are with the victims of this horrific attack, their families and friends".
He added: "The police and security staff have taken swift action to ensure the safety of the public, MPs and staff, and we are grateful to them."
Mayor of London Sadiq Khan also released a statement, saying:
"There has been a serious incident near to Parliament Square this afternoon which is being treated as a terrorist attack until the police know otherwise.
"I have spoken to the Acting Commissioner. The Metropolitan Police Service is dealing with the incident and an urgent investigation is underway. My thoughts are with those affected and their families.
"I would like to express any thanks to the police and emergency services who work so hard to keep us safe and show tremendous bravery in exceptionally difficult circumstances."
The incident happened on the one year anniversary of the Brussels bombings, which killed 32 people and wounded more than 300.
In Scotland, MSPs have delayed a vote on a possible second independence referendum in light of the attack in Westminster. The Scottish Parliament and Welsh Assembly were both suspended for the day.
American journalist Bradd Jaffy posted an immediate reaction from US President Donald Trump to the news:
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