LONDON — An assailant fatally stabbed a police officer at the gates to Britain’s Parliament compound Wednesday after plowing a vehicle through terrified pedestrians along a landmark bridge. The attacker was shot and killed by police, but not before claiming a total of three lives in what appeared to be Europe’s latest high-profile terrorist attack.

Personal details about the suspected attacker were not immediately made public. Police said the man traced a deadly path across the Westminster Bridge, running down people with an SUV, then ramming the vehicle into the fence encircling Parliament. More than 20 people were reported injured.

[As the London attacks show, the sedate sedan can be turned into a deadly, unpredictable weapon]

Finally, the attacker charged with a knife at officers stationed at the iron gates leading to the Parliament grounds, authorities said.

The dead and injured were left scattered on some of London’s most famous streets.

Crumpled bodies lay on the Westminster Bridge over the River Thames, including at least two people killed. Outside Parliament, a Foreign Office minister — covered in the blood of the stabbed police officer — tried in vain to save his life.

Parliament chambers and offices were put on full lockdown for more than two hours and officials shut down the famous London Eye Ferris wheel, which overlooked the scene.

“This is a day that we planned for but hoped would never happen. Sadly it has now become a reality,” said the assistant Metropolitan Police commissioner, Mark Rowley, outside Scotland Yard’s headquarters.

As he spoke, the bells of Big Ben tolled six times to mark the hour.

Later, after chairing a meeting of the government’s emergency committee, British Prime Minister Theresa May said: “The location of this attack was no accident. The terrorist chose to strike at the heart of our capital city, where people of all nationalities, religions and cultures come together to celebrate the values of liberty, democracy and freedom of speech.”

But “any attempt to defeat those values through violence and terror is doomed to failure,” she vowed, adding: “Tomorrow morning, Parliament will meet as normal.”

Even before full details emerged, the attack and resulting chaos appeared certain to raise security levels in London and other Western capitals and bring further scrutiny to counterterrorism measures.

“We are treating this as a terrorist incident until we know otherwise,” said a Twitter message from London Metropolitan Police.

[In Europe, terror fears are now fact of life]

The attack occurred on Parliament’s busiest day of the week, when the prime minister appears for her weekly questions session and the House of Commons is packed with visitors.

The Palace of Westminster, the ancient seat of the British Parliament, is surrounded by heavy security, with high walls, armed officers and metal detectors. But just outside the compound are busy roads packed with cars and pedestrians.

The attack — a low-tech, high-profile assault on the most potent symbol of British democracy — fits the profile of earlier strikes in major European capitals that have raised threat levels across the continent in recent years.

It was apparently carried out by a lone assailant who used easily available weapons to attack and kill victims in a busy, public setting.

British security officials have taken pride in their record of disrupting such attacks even as assailants in continental Europe have slipped through. But they have also acknowledged that their track record would not stay pristine, and that an attack was inevitable.

When it happened, it was shocking nonetheless. Cellphones captured scenes of carnage amid some of London’s most renowned landmarks.

The target — Westminster — was heavily guarded. But the weapons of choice — an SUV and a knife — made the attack difficult to prevent, requiring the assailant neither to acquire illegal weapons nor to plot with other conspirators.

Rowley said investigators believe that just one assailant carried out the attack, but he encouraged the public to remain vigilant.

Britain has been on high alert for terrorist attacks for several years. But until Wednesday, the country had been spared the sort of mass-casualty attacks that have afflicted France, Belgium and Germany since 2015.

David Lidington, a member of Parliament, said a police officer was stabbed and the suspected assailant was shot.

[Germany’s new strategy for homegrown terror suspects? Deportation.]

“Suddenly police cars drove down the road and locked it down. People threw themselves to the ground and hid behind trash cans, walls and in cafes. But the situation seemed to be under control fairly quickly,” said Lee Stevens, 34, who was standing outside Downing Street, about 500 yards from Parliament and near the prime minister’s offices.

Among those providing emergency aid was Tobias Ellwood, a senior official at the Foreign Office and a British military veteran. Photos showed Ellwood’s face streaked with blood after attempting to revive a police officer who had been stabbed just inside the gates of the parliamentary compound.

French Prime Minister Bernard Cazeneuve said that among those wounded in the vehicle attack were members of a group of French students. News media in France reported that three of the students, on a school trip from a high school in Brittany, were in serious condition and that their parents were being flown to London immediately.

King’s College Hospital in south London tweeted Wednesday evening that its emergency department was treating eight victims, two of whom were in critical condition.

In Washington, White House press secretary Sean Spicer said President Trump has been briefed on the London attack and spoke by phone with Prime Minister May. 

“We condemn today’s attack in Westminster,” Spicer told reporters. “We applaud the quick response of the British police and the first responders.” He pledged “the full support of the U.S. government in responding to the attack and bringing to justice those who are responsible.”

May later chaired a meeting of the government’s emergency committee to discuss the assault.

Raffaello Pantucci, director of international security studies at the Royal United Services Institute think tank, said the rapid response suggested that police “were expecting that an attack was highly likely for some time.”

Images from the bridge showed a man dressed in a suit lying on his back, his legs splayed to either side, as pedestrians huddled around him administering first aid. The shoe was off his right foot, and blood stained the sidewalk beneath his left.

In another image, a woman with long blond hair and running shoes lay in a pool of blood on the bridge’s sidewalk. Blood stained the corner of her mouth as another pedestrian cradled her head.

Other photos showed people sitting on the sidewalk looking dazed amid broken glass and bits of automotive debris, with Big Ben looming beyond.

A spokesman for the Port of London Authority said a woman was pulled alive from the River Thames, and he confirmed reports that she had serious injuries.

As police investigated, much of the activity in the area around Westminster came to a standstill.

A nearby hospital was put on lockdown and the London Eye — the enormous Ferris wheel above the River Thames — was stopped and visitors were slowly let off hours later. Those who were locked inside the Eye’s capsules at the time of the attack were kept there, hovering above as emergency responders swarmed the scene below.

In a brief news conference just before 5 p.m. outside the nearby headquarters of Scotland Yard — London’s police force — a spokesman said he was “not going to speculate” on whether the incident was over.

Another witness, Kirsten Hurrell, 70, said she first heard the crash of a car hitting the fence outside parliament.

“I thought initially it was some kind of accident,” Hurrell told the Guardian newspaper. “Then I heard a couple of sharp noises. It could have been gunshots. I wasn’t sure.”

“There was a lot of steam from the car,” added Hurrell, who runs a newspaper kiosk in Parliament Square. “I thought it might explode.”

The U.S. Department of Homeland Security said it was in “close contact with our British counterparts to monitor the tragic events and to support the ongoing investigation.” It noted that U.S. security threat levels remained unchanged.

A year ago to the day, attackers carried out three coordinated suicide bombings in Belgium, killing 32 civilians and injuring more than 300 others in two blasts at Brussels Airport and one at a metro station in the Belgian capital. The Islamic State claimed responsibility for the attacks, in which three perpetrators were also killed. Another bomb that failed to explode was found at the airport.

The attacks occurred shortly after Belgian police staged a series of raids targeting suspected terrorists. Those who carried out the bombings belonged to a cell that was involved in a series of gun and bomb attacks that killed 130 people in Paris in November 2015. The Islamic State also claimed responsibility for the Paris attacks.

In the aftermath of the attack in London, the Welsh Assembly and the Scottish Parliament both suspended their sessions. Scottish lawmakers had been due to debate legislation authorizing a new referendum on independence.

In July last year, a Tunisian resident of France perpetrated a new type of terrorist attack in the Riviera city of Nice, using a cargo truck to mow down revelers celebrating Bastille Day on a seaside promenade. Eighty-six people were killed and more than 400 injured before the driver was fatally shot by police. The Islamic State said the attacker was “a soldier” of the group who responded to its calls to use all means, including vehicles, to strike “behind enemy lines.”

The Nice attack apparently served as a template for another truck assault in December, when a Tunisian who had sought asylum in Germany plowed into a Christmas market in Berlin, killing 12 people and injuring more than 50 others before fleeing. The attacker, who had pledged allegiance to the Islamic State’s leader, was later killed by Italian police in a shootout near Milan.

In October 2014, a Canadian of Libyan heritage went on a shooting spree at the Parliament building in Ottawa, killing a soldier on sentry duty and engaging in a shootout with parliamentary security guards in what police described as a terrorist act. The attacker was fatally shot at the scene.

Specialists said the London attack Wednesday appeared to be in line with an emerging model of strikes involving simple, everyday instruments but carried out in locations sure to draw global attention.

“Terrorists rely on a lot of people watching — it can be even better than having a lot of people dead,” said Frank Foley, a scholar of terrorism and counterterrorism at the Department of War Studies at King’s College London.

Strict regulation of firearms in Britain — as compared to the United States, where such attacks have often involved gunfire — lowers the scale of possible violence, said Steve Hewitt, who studies surveillance and counterterrorism at the University of Birmingham.

“We live in a country where there are tight gun-control laws, as opposed to in the U.S., where a lone individual acquiring weapon often legally can cause major death and destruction very quickly,” Hewitt said.

Within a few hours of the attack, there were signs that normalcy was returning to London. Alongside police officers and journalists near Westminster were large numbers of tourists who had come to visit sites in the now cordoned-off area or just outside it.

At the London Eye near the Westminster Bridge, a large crowd of tourists stood by the ferris wheel. Some were waiting for friends and relatives to get off the ride, which was halted when the attack occurred, while others had turned up unaware of the commotion or had come to watch.

Linda Lim, a 22-year-old student from Chicago, had just arrived at the scene. While she had heard about the attack, she did not realize it happened so close to the London Eye and had not expected the crowds of police officers.

Charles Thompson, a 21-year-old chef from Canada, wondered if there would be more attacks. “Usually its a chain-reaction thing,” he said.

His friend, Enrique Cooper, a 32-year-old officer manager originally from Italy, said he would not let the day’s violence change his view of London. “I’m here all the time,” he said. “You can’t let something like this ruin your perspective.”

Witte reported from Madrid. Adam Taylor and Isaac Stanley-Becker in London, James McAuley in Paris and Brian Murphy, William Branigin and Mark Berman in Washington contributed to this report.

Read more:

In the London attack, one MP tried desperately to save a police officer

Watch: Trump cites Paris in warnings on terrorism

Today’s coverage from Post correspondents around the world

Like Washington Post World on Facebook and stay updated on foreign news