Jane Onyanga-Omara

USA TODAY

LONDON — A terrorist hurtled a car into one of the world's most famous thoroughfares Wednesday, fatally smashing into horrified pedestrians before hopping out and killing a police officer near Britain's Parliament. He killed a total of four, including three civilians, before being shot to death by police.

The attack began mid-afternoon in the heart of London when the killer, who police had not yet identified, used his car to mow down several people on the Westminster Bridge. At least 40 were hurt, including some with "catastrophic injuries," police said. The driver then rammed the car into the gates outside Parliament.

London's Metropolitan Police said they would treat the attack "as a terrorist incident until we know otherwise."

"Although we remain open minded to the motive, a full counterterrorism investigation is already underway — this is led by the Met's Counter Terrorism Command," Commander B.J. Harrington of New Scotland Yard said.

The attack came on a particularly busy day in London. Tourists crowded the historic sites, and most members of Parliament had gathered in the House of Commons for Prime Minister's Questions, a 30-minute session of questions with Prime Minister Theresa May that takes place every Wednesday at noon.

Local media reported that May is safe after security officials whisked her from the scene as shots were heard.

"The location of this attack was no accident. The terrorists 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," May said in statement Wednesday. "Any attempt to defeat those values through violence and terror is doomed to failure."

May said Parliament will meet Thursday.

London Mayor Sadiq Khan echoed May's sentiments, saying, "There will be additional armed and unarmed police officers on our streets from tonight in order to keep Londoners and all those visiting our city safe."

"Londoners will never be cowed by terrorism,” Khan said in a statement.

The murdered police officer was identified as Keith Palmer, a member of the Parliamentary and Diplomatic Protection Command.

"Keith - aged 48 - had 15 years' service and was a husband and a father," said Mark Rowley, the national lead for Counter Terrorism Policing and the Acting Deputy Commissioner of the Metropolitan Police. "He was someone who left for work today expecting to return home at the end of his shift and he had every right to expect that would happen."

Rowley said the working assumption is that the attacker was motivated by "international terrorism."

"Terrorists have a clear aim and that is to create discord, distrust and to create fear," he said. "The police stand with all communities in the UK and will take action against anyone who seeks to undermine society, especially where their crimes are motivated by hate. We must recognize now that our Muslim communities will feel anxious at this time given the past behaviour of the extreme right wing and we will continue to work with all community leaders in the coming days."

France's prime minister said some people injured on the bridge were French high school students.

Colleen Anderson, a doctor at St Thomas’ Hospital, in an interview with the Associated Press, confirmed the death of a female pedestrian.

"There were people across the bridge," Anderson told the AP. "There were some with minor injuries, some catastrophic. Some had injuries they could walk away from or who have life-changing injuries."

Port officials said they pulled a woman from the Thames River, injured but still alive.

The assault comes on the one-year anniversary of the Brussels suicide bomb attacks that killed 32 people and wounded hundreds. It is the first serious terrorist incident in Britain since 2013, when an off-duty soldier named Lee Rigby was killed by two al-Qaeda-inspired extremists.

Since 2013, British counterterrorism forces have thwarted 13 potential terrorist attacks. The assault on the bridge was the latest in a string of incidents, including in Berlin and Nice, France, in which attackers have used vehicles as weapons.

Read more:

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London terror attack near the UK Parliament: What we know now

Quentin Letts, a journalist at the Daily Mail whose office overlooks the attack scene, told the BBC: The attacker "had something in his hand. It looked like a stick of some sort and he was challenged by a couple of policemen in yellow jackets and one of the yellow-jacketed policemen fell down.

"We could see the man in black moving his arm in a way that suggested he was either stabbing or striking the yellow-jacketed policeman, and one of the policemen then ran to get help, which was very quick to come and as this attacker was running toward the entrance used by (parliamentarians) to get into the House of Commons.

"As he was running — he ran about I'd say 15 yards perhaps — two plainclothes guys with guns shouted at him what sounded like a warning. He ignored it and they shot two or three times and he fell."

Office workers negotiated police cordons as they headed home for the day, using alternative routes because the Westminster subway station, which sits directly opposite Parliament, was closed for security reasons. Police officers acted as guides for tourists and those confused about how to reach their destinations.

"I think it's shocking but for me, it's not surprising, said Vanessa Fischer, 21, visiting London from Germany. She added that big cities were often a target for attacks.

"I don't associate these kinds of scenes with London," said Alexander Castro, 33, an IT engineer on his way home Wednesday. "We take security for granted."

Queen Elizabeth II’s visit to open Scotland Yard’s new headquarters Thursday in London is being postponed, Buckingham Palace announced to British media late Wednesday. The queen, 90, and her husband, Prince Philip the Duke of Edinburgh, 95, had been scheduled to visit the new Yard headquarters on the Victoria Embankment, nearby to Westminster. The visit is one of hundreds of engagements the royal couple undertake every year.

“In light of today's events the decision has been taken to postpone The Queen's engagement to New Scotland Yard tomorrow. The visit will be rearranged for a later date," a Buckingham Palace statement said.

Earlier in the day, before the suspected terrorist attack, the queen went about her routine as usual, holding a private audience at the palace to accept the credentials of Thailand’s new ambassador to the U.K.

Contributing: Maria Puente