At least four people were killed as a knife-wielding assailant driving a sport utility vehicle mowed down pedestrians and stabbed a police officer outside the British Parliament on Wednesday, according to media sources.

London police shot the assailant dead while 40 others were injured in what Prime Minister Theresa May condemned as a "sick and depraved terrorist attack”, reported New York Times.

“The full details of exactly what happened are still emerging,” she added, confirming that the attack had been carried out by a lone male assailant. As of late Wednesday, his identity had not been released, but Scotland Yard officials said they believed they knew who he was, according to the report.

Ambulances, emergency vehicles and heavily armed security officers thronged the area outside Parliament during the afternoon, as one of the busiest sections of London was cordoned off and evacuated.

Prime Minister Theresa May was rushed into a vehicle and spirited back to her office. She held a meeting of the government’s emergency committee and issued a statement on Wednesday night from her 10 Downing Street residence denouncing “the sick and depraved terrorist attack on the streets of our Capital this afternoon.”

The attack unfolded around 2:40 pm, assistant commissioner Mark Rowley said at a news conference.

Driving a large sport utility vehicle, the assailant slammed into pedestrians on Westminster Bridge near Parliament, killing two people and injuring many others, before crashing into a railing. A third person injured on the bridge later died at a hospital.

After the crash, the driver left the vehicle and approached Parliament, where he stabbed an armed police officer to death and was fatally shot by the police.