There are reports on social media of shots being fired outside the Houses of Parliament in Westminster, London.
Witnesses have posted videos and photographs on Twitter of people with wounds being treated. A number of witnesses also reported a car being driven across Westminster Bridge that "mowed down" at least five people.
An air ambulance has landed outside Parliament, and people inside have tweeted that they have been instructed to stay where they are. All activities of the houses of Parliament have been suspended.
George Eaton, the political editor of the New Statesman, witnessed the attack from a window in Parliament. He said:
I have just seen police shoot a man who charged at officers while carrying what appeared to be a knife. A large crowd was seen fleeing the man before he entered the parliamentary estate. After several officers evaded him he was swiftly shot by armed police. Ministers have been evacuated and journalists ordered to remain at their desks.
House of Commons leader David Lidington has since confirmed to MPs that a "police officer has been stabbed" and the "alleged assailant was shot by armed police."
11:45 AM update
The Guardian is reporting that Theresa May was ushered into a car by at least eight armed officers. She has been confirmed safe.
The Metropolitan Police have tweeted that they are treating this as a terror attack.
The BBC has picture of the car that hit people on Westminster Bridge before crashing:
Police are asking people to avoid Parliament Square, Whitehall, Westminster, and Lambeth Bridges, as well as Victoria Street.
There are reports of between five and twelve people injured, but it's currently unclear if that includes the attackers or members of the police. A number of journalists report a blanket being put over one person who has since stopped being treated by First Aid. The Guardian is reporting that "it seems certain they are dead."
12:15 PM update:
A junior doctor at St. Thomas's Hospital has told the Press Association that one woman has died and others are hurt, some with "catastrophic" injuries.
Police are yet to give an official number, but fear there are more than ten people injured.
12:50 PM update
Sky and a number of other sources are now reporting that two people have died.
ITV is reporting that one woman was pulled from the River Thames and paramedics are now treating her.
Tory MP Tobias Elwood gave mouth-to-mouth resuscitation to the police officer who was stabbed and waited with him until the air ambulance arrived.
The Metropolitan Police have made their first statement about the attack.
The far-right activist Tommy Robinson, who was in Westminster at the time, is now ranting to assembled media, with another member of the far right.
1:35 PM update
It has been confirmed by Tory MP Tobias Elwood that the police officer who was stabbed by the attackers has died. Elwood, a government minister, tried to save his life at the scene by giving mouth-to-mouth resuscitation.
The Port of London Authority has confirmed that a woman was pulled from the River Thames.
Spokesman Martin Garside said:
A female member of the public was recovered alive from the water, but with serious injuries. She has been brought ashore and is undergoing urgent medical treatment. The working assumption is that she fell or jumped from the bridge.
2:10 PM update
The man believed to be the attacker is dead, according to the police. Four people, including the attacker, have now been confirmed dead as result of the attack. Twenty people have been injured.
The police say "at this stage it looks like there was only one attacker." A number of outlets have run a picture of the man believed to be the attacker.
4:00 PM update
White House spokesperson Sean Spicer has told press: "We obviously condemn today's attack in Westminster, which the United Kingdom is treating as an act of terrorism."
President Donald Trump followed this statement with a tweet, offering his condolences to the UK.
London mayor Sadiq Khan has said there will be extra armed and unarmed police patrolling the streets of London Wednesday night, but urged residents and tourists not to worry. "Londoners will never be cowed by terrorism," he said.
5:20 PM update
In a press conference, Prime Minister Theresa May described the attacks as "sick and depraved." She hailed the "exceptional bravery of our police and security services," and said "the values our Parliament represents—democracy, freedom, human rights, the rule of law" is why it was attacked. "But let me make it clear today," she added, "as I have had cause to do before: any attempt to defeat those values through violence and terror is doomed to failure."
The situation is ongoing and we will update this story as we know more