LONDON (AP) — A vehicle mowed down pedestrians on London's Westminster Bridge, killing at least one woman and leaving others with injuries described as catastrophic.

Around the same time Wednesday, a knife-wielding attacker stabbed a police officer and was shot on the grounds outside Britain's Parliament, sending the compound into lockdown. Authorities said they were treating the attacks as a terrorist incident.

The threat level for international terrorism in the U.K. was already listed at severe. Wednesday was the anniversary of suicide bombings in the Brussels airport and subway that killed 32 people, and the latest events echoed recent vehicle attacks in Berlin and Nice, France. There was no immediate claim of responsibility and it was not clear if there was more than one attacker. London Police Commander B.J. Harrington said a full counter-terrorism investigation was underway.

The incident in London unfolded within sight of some of the city's most famous tourist sites, including the London Eye, a large Ferris wheel with pods that have views over the capital. It stopped rotating and footage showed the pods full as viewers watched police and medical crews on the bridge, which has at its north end Big Ben and Parliament, two iconic symbols.

"The whole length of the bridge there were people on the ground," Richard Tice, a witness, told Sky News. The London Ambulance Service said it had treated at least 10 people on the bridge.

Colleen Anderson of St. Thomas' Hospital said a female pedestrian died and around a dozen people were hurt.

"There were some with minor injuries, some catastrophic. Some had injuries they could walk away from or who have life-changing injuries," she said.

Rick Longley told the Press Association that he heard a bang and saw a car plow into pedestrians and come to a crashing stop. Images from the scene showed pedestrians sprawled on the ground, with blood streaming from a woman surrounded by a scattering of postcards.

"They were just laying there and then the whole crowd just surged around the corner by the gates just opposite Big Ben," he said. "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."

At Parliament, a body was seen lying in the yard. It wasn't clear if it was the attacker.