WATCH | Here's a break down of the deadly attack, Sara Carter from Circa's newsroom
What we know so far
A car drove over London's Westminster Bridge, hitting multiple pedestrians before crashing into railings. Members of Parliament said a police officer was stabbed before the assailant was shot. London police said they are treating the incident as a terrorist attack "until we know otherwise." Prime Minister Theresa May is safe. Police reported four deaths, including the alleged attacker and stabbed police officer.
Live updates below
UPDATE 9:12p.m. EST:
The number of casualties has been updated to five, according to the Associated Press. Forty others were injured in what Prime Minister Teresa May condemned as a "sick and depraved terrorist attack."
WATCH | Here's what we know in video form.
UPDATE 3:52 p.m. EST:
Members of Parliament are making the journey home after being held on lockdown for four hours in the Parliament building.
Here's our live feed of coverage on the incident at the British Parliament.
Released from being locked sown in the Chamber. My thoughts are with the two people including the policeman who died and others injured— John Howell (@JHowellUK) March 22, 2017
We have just been released from Parliament and starting journey home. Terrible day for so many families— Stephen McPartland (@SMcPartland) March 22, 2017
UPDATE 3:36 p.m. EST:
May ordered flags lowered to half-mast over Downing Street to pay tribute to the victims who lost their lives in the attack, the BBC reported.
UPDATE 2:27 p.m. EST:
White House Press Secretary Sean Spicer said that it was irresponsible for the U.S. to "get ahead" of the U.K. investigation into the Parliament attack.
A European government official told Reuters that investigators were considering the possibility the London attack was inspired by ISIS propaganda.
MORE: UK investigators considering possibility that London attack was inspired by Islamic State propaganda - European government official— Reuters Top News (@Reuters) March 22, 2017
UPDATE 2:00 p.m. EST:
A Scotland Yard spokesman confirmed four people have died as a result of the attack on Parliament, including a police officer and the attacker. At least 20 others were injured.
The spokesman also confirmed the incident as a terrorist attack. Only one attacker is believed to have been involved at this time.
BREAKING: British police say that 4 people dead in London attack including 1 police officer, attacker; 20 injured.— The Associated Press (@AP) March 22, 2017
UPDATE 1:55 p.m. EST:
The police officer stabbed outside the Parliament building has died, the BBC reports. At least one other person has died.
UPDATE 1:42 p.m. EST:
White House Press Secretary Sean Spicer said the U.S. condemned the incident in Parliament, calling it an act of terrorism.
Spicer also said President Trump had spoken to British Prime Minister Theresa May about the incident.
Here's the U.S. Department of Homeland Security statement: "The Department of Homeland Security stands in solidarity alongside our friends in the United Kingdom in condemning the terrible attack at Parliament today. With our partners in federal law enforcement, we are in close contact with our British counterparts to monitor the tragic events and to support the ongoing investigation. At this time our domestic security posture remains unchanged. However, our frontline officers and agents continue to stay vigilant in safeguarding the American people and our homeland."
UPDATE 1:36 p.m. EST:
French Prime Minister Bernard Cazeneuve tweeted that French students were among the injured in the attack on Parliament.
He expressed solidarity with "our British friends" on Twitter.
Solidarité avec nos amis britanniques terriblement frappés, plein soutien aux élèves français blessés, à leurs familles et leurs camarades.— Bernard Cazeneuve (@BCazeneuve) March 22, 2017
UPDATE 1:06 p.m. EST:
Witnesses described the attacks as "absolutely horrendous." One woman said she could hear a woman screaming underneath a car's wheels.
The leaders of the House of Commons and House of Lords published the following statement after the incident:
"An extremely serious incident has occurred in the Westminster area this afternoon. The Metropolitan Police is dealing with this and an investigation is underway. On behalf of Members of both Houses of Parliament, we wish to offer our thoughts to all those affected and their families. We would also like to that express our gratitude to the police and all emergency services.”
A woman was recovered from the water under the bridge alive, a Port of London Authority spokesman said.
UPDATE 12:56 p.m. EST:
This video shows the car that crashed into Westminster Bridge.
Here's the law enforcement press conference mentioned below.
CNN reported Buckingham Palace, where the Queen is currently, has been closed as a precaution.
UPDATE: 12:50 p.m. EST:
The Telegraph reports Tobias Elwood, a member of Parliament, performed CPR to try to save the life of the police officer who was stabbed inside Parliament.
Elwood, a former soldier, was reportedly seen with bloody hands talking to officers nearby.
Meanwhile, The Guardian reports some members of Parliament and visitors were led out of the building through Westminster Hall.
UPDATE 12:46 p.m. EST:
Commander B.J. Harrington of London Metropolitan Police told reporters that there were a "number" of casualties in the incident, including law enforcement officers, but did not give a specific number.
When asked if the incident was over, he merely said officers had responded. He did not respond to questions of whether or not gunshots were fired.
UPDATE 12:38 p.m. EST: Here's London Mayor Sadiq Khan's statement.
UPDATE 12:34 p.m. EST:
Members of Parliament remain on lockdown and have been told to stay away from windows.
Those injured in the incident are being flown to nearby hospitals.
UPDATE 12:30 p.m.: One photo showed a police officer standing on a knife while pointing a gun.
The Guardian reported a debate in the Scottish Parliament regarding a possible referendum to leave the United Kingdom was suspended out of respect for the "sister parliament."
WATCH | Here are President Trump's brief comments on the London incident before a meeting with women in the health care field Wednesday.
UPDATE 12:15 p.m.:
The BBC reports at least one woman has died and many have been injured, some with "catastrophic" injuries, according to a doctor at St. Thomas' Hospital.
UPDATE 12:12 p.m.:
Facebook has activated its Safety Check feature in response to the terrorist incident in London, CNN reports.
UPDATE 12:02 p.m.:
The BBC reports one man both drove the car and stabbed a police officer. He reportedly hit several pedestrians while driving a gray Hyundai across Westminster Bridge. He then ran through the gates of the Parliament building and stabbed an officer before getting shot as he approached a second officer.
We have declared a major incident and our priority is to assess patients and ensure they are treated and taken to hospital as soon as possible.
UPDATE 12:00 p.m.: London Ambulance Services published a statement advising people not to call except in a "genuine emergency" as many teams were dedicated to treating the injured near Parliament.
The Westminster station of the London Underground has been closed. City police asked the public to avoid the area and to share any images and video they had.
Today also marks the one-year anniversary of ISIS' deadly attack in Brussels that killed 32 people.
Let's remember today is the anniversary of the Brussels attack. Events in front of Parliament could have symbolic gesture.— Julie Lenarz (@MsJulieLenarz) March 22, 2017
Poland's former foreign minister said he saw a car mow down "at least five people."
UPDATE 11:30 a.m.: This video shows the moment Parliament was suspended.
UPDATE 11:20 a.m.:
Kevin Brennan, a member of Parliament who was in the building for the lockdown, told people he had heard that a police officer within Parliament was stabbed, and the attacker was shot.
He also said he had heard rumors of another incident. Some witnesses have said a car tried to mow down pedestrians in the area, and some photos showed a car crash near Westminster Bridge.
CNN reported President Trump was briefed on the incident.
Britain's intelligence agency, MI5, said it was too early to say if terrorism was involved.
UPDATE 11:13 a.m.: Prime Minister Theresa May was reported safe.
There was a loud bang. Screams. Commotion. Then the sound of gunshots. Armed police everywhere.— Tom Peck (@tompeck) March 22, 2017
Multiple witnesses said they saw a car try to run people down.
UPDATE 11:09 a.m.: This claims to show police lined up outside the Palace of Westminster.
One witness said he saw "bodies strewn everywhere" on Westminster Bridge.
MORE: Policeman stabbed inside parliament - leader of House of Commons— Reuters Top News (@Reuters) March 22, 2017
The Leader of the House of Commons said a police officer was stabbed inside Parliament.
UPDATE 11:08 a.m.: The U.S. State Department warned travelers to stay vigilant.
UPDATE 11:03 a.m.:
The alleged attacker was shot at the Parliament building by British police, Reuters reports.
A Reuters photographer reported hearing loud bangs outside the Parliament building.
A CNN reporter said Parliament was in session at the time of the incident.
GRAPHIC: This video purports to show wounded victims of the attacker.
ORIGINAL STORY: Two people have reportedly been shot outside the United Kingdom's Parliament building, Reuters reports.
The building has been locked down.
The BBC reported witnesses saw a man with a knife roaming the palace grounds.
LATEST: At least a dozen injured people on Westminster Bridge near UK parliament: Reuters photographer— Reuters Top News (@Reuters) March 22, 2017
A Reuters photographer reported at least a dozen people were injured nearby.
MORE: British police say incident on Westminster Bridge being treated as firearms incident— Reuters Top News (@Reuters) March 22, 2017
The bridge incident is being treated as a "firearms incident."