officer to death inside the gates of Parliament. Four people were killed, including the attacker, and about 20 others were injured in what authorities called a terrorist incident.
Lawmakers, lords, staff and visitors were locked down after the man was shot by police within the perimeter of Parliament, just yards (meters) from entrances to the building itself and in the shadow of the iconic Big Ben clock tower. He died, as did two pedestrians on the bridge, and the police officer.
A doctor who treated the wounded from the bridge said some had "catastrophic" injuries. French teenagers on a school trip and two Romanian tourists were among the casualties.
Police said they were treating the attack as a terrorist incident and had launched a full counterterrorism investigation. There was no immediate claim of responsibility.
"We are satisfied at this stage that it looks like there was only one attacker," said Metropolitan Police counterterrorism chief Mark Rowley. "But it would be foolish to be overconfident early on."
The threat level for international terrorism in the U.K. was already listed at severe, meaning an attack is "highly likely."
Wednesday was the anniversary of suicide bombings in the Brussels airport and subway that killed 32 people last year, and the latest events echoed recent vehicle attacks in Berlin and Nice, France.
In the House of Commons, legislators were holding a series of votes on pensions when deputy Speaker Lindsay Hoyle announced that the sitting was being suspended and told lawmakers not to leave.
Parliament was locked down for several hours, and adjoining Westminster subway station was shuttered.
Conservative lawmaker Tobias Ellwood, whose brother was killed in the Bali terror attack in 2002, performed first aid on the wounded police officer, who later died. About 10 yards away lay the assailant.
"I tried to stem the flow of blood and give mouth to mouth while waiting for the medics to arrive but I think he had lost too much blood," Ellwood said. "He had multiple wounds, under the arm and in the back."
The attack began early Wednesday afternoon, as a driver in a gray SUV slammed into pedestrians on the bridge.
Former Polish foreign minister Radek Sikorski was in a car crossing the bridge when he heard "something like a car hitting metal sheet" and then saw people lying on the pavement.
"I saw one person who gave no signs of life. One man was bleeding from his head. I saw five people who were at least seriously injured," Sikorski told Poland's TVN24.
Ambulances arrived within minutes to treat people who lay scattered along the length of the bridge, which links Parliament to the south bank of the River Thames. One bloodied woman lay surrounded by a scattering of postcards.
Police said one injured woman was pulled from the river.
The car crashed into railings on the north side of the bridge, less than 200 yards (meters) from the entrance to Parliament. As people scattered in panic, witnesses saw a man holding a knife run toward the building.
"The whole crowd just surged around the corner by the gates just opposite Big Ben," said witness Rick Longley. "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."
The attacker managed to get past a gate into Parliament's fenced-in New Palace Yard, a cobbled courtyard in the shadow of the Big Ben clock tower.
Daily Mail journalist Quentin Letts said a man in black attacked the police officer before being shot two or three times as he tried to storm into the building.
"As this attacker was running towards the entrance two plain-clothed guys with guns shouted at him what sounded like a warning, he ignored it and they shot two or three times and he fell," Letts told the BBC.
The attacker fell to the cobbles just yards from the entrance to 1,000-year-old Westminster Hall, the oldest part of the parliamentary complex, busy with visitors and school groups. Beyond that, a corridor leads to the building's Central Lobby, flanked by House of Commons and House of Lords chambers.
Prime Minister Theresa May was among lawmakers near the Commons at the time of the attack, and was quickly ushered away by security officers and driven back to Downing Street.
To get that far, the attacker would have had to evade the armed officers who patrol the Parliament complex in pairs, as well as Parliament's own security staff, who don't carry guns.
The attack unfolded near some of the city's most famous tourist sites, including the London Eye, a large Ferris wheel with pods that overlook the capital. It was halted after the attack, stranding visitors in the pods, with an aerial view of the attack scene.
London Ambulance Service said medics treated 12 people for serious injuries and eight who were less seriously hurt.
Dr. Colleen Anderson of St. Thomas' Hospital said some of the wounded had "catastrophic" injuries.
"Some had injuries they could walk away from or who have life-changing injuries," she said.
The French Foreign Ministry said that three students on a school trip from Saint-Joseph in the Brittany town of Concarneau were among the injured. Two Romanians were also among the injured, the country's Foreign Ministry said.
French Prime Minister Bernard Cazeneuve offered support to the British and to "the French students wounded, their families and their schoolmates."
U.S. President Donald Trump spoke with May by telephone and applauded "the quick response of British police and first responders," spokesman Sean Spicer said.
London has often been the target of terrorist attacks, from IRA campaigns in the 1970s and 80s to more recent Islamist plots.
On July 7, 2005, four al-Qaida-inspired British bombers blew themselves up on three subway trains and a bus in London, killing 52 people.
British security forces say they have thwarted some 13 terror plots over the past four years, but in recent years the U.K. has largely been spared major international terror attacks such as the ones seen in Belgium and France.
Last year, a far-right supporter shot and killed British lawmaker Jo Cox, who had campaigned for the U.K. to remain in the European Union. Prior to that, an attacker claiming to be motivated by Syria stabbed three people at a London subway station.
The most gruesome recent attack occurred in 2013 when two Muslim converts of Nigerian descent attacked Lee Rigby, a British soldier who was walking down the street. The men ran Rigby down with their vehicle and then used a cleaver to hack him to death as bystanders watched in horror.