An apparent attack outside Parliament plunged London into turmoil on Wednesday, leaving at least four people including a police officer dead and 20 injured, and prompting the hasty evacuation of the prime minister in a confusing swirl of violence that traumatized the seat of British power, reports the New York Times .

The police said they were treating the attack as terrorism, which appeared to make it the most serious such assault in London since the deadly subway bombings more than a decade ago.

The attack unfolded around 2:40 p.m., Assistant Commissioner Mark Rowley said at a news conference.

The driver of a large vehicle mowed down pedestrians on Westminster Bridge, which leads to Parliament, killing two people and injuring others, before crashing into a railing.

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

Tobias Ellwood, a minister in the Foreign Office, tried to save the life of the stabbed police officer by giving mouth-to-mouth resuscitation, but the officer died.

"A full counterterrorism investigation is underway," Cmdr. B. J. Harrington of the Metropolitan Police said at a brief news conference. Commander Harrington said that the police knew of "a number of casualties, including police officers," but that he could not confirm a precise figure. At least three police officers were among the 20 people injured.