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

Around the same time, 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. Today 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 under way.

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.

British port officials say they pulled a woman from the Thames River, injured but alive after the attack.

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

Daily Mail journalist Quentin Letts said he saw a man in black attack a police officer outside Parliament before being shot two or three times as he tried to storm into the House of Commons.

Prime Minister Theresa May was at Parliament when the incident began, but her office says she is safe.

British security has thwarted some 13 terror plots over the past four years, but the UK has largely been spared major international terror attacks such as the ones seen in Belgium and France.