The sun sets over London on Wednesday after the UK's Houses of Parliament became a terrorist attack target. Photo: Reuters/Hannah McKay

Amid rumour, confusion but little panic, terrorism returned to the streets of London on Wednesday. A police officer was among the five killed in the attack outside Britain’s parliament by an assailant who was then shot dead by armed police, the top anti-terror officer said.

Mark Rowley said the two other victims died on Westminster Bridge in London after the attacker mowed down pedestrians in a car, leaving at least 20 people injured in total.

“Sadly I can confirm that now four people have died,” the Metropolitan Police assistant commissioner told reporters outside Scotland Yard police headquarters.

“That includes the police officer who was protecting parliament and one man who we believe to be the attacker, who was shot by a police firearms officer.

“We have declared this as a terrorist incident and the counter-terrorism command are carrying out a full-scale investigation into the attacks.”

He said the assault began when a car was driven over Westminster Bridge next to the Houses of Parliament, hitting and injuring several members of the public, including three police officers returning from a ceremony.

“The car then crashed near to parliament and at least one man, armed with a knife, continued the attack and tried to enter parliament,” Rowley said.

He said the police implemented “well-rehearsed plans… for a marauding terrorist attack” and parliament was put into “lockdown”.

Rowley said a “massive operation” was now under way but refused to go into further details.

“We currently believe there was only one attacker,” he said.

With the UK’s national newspapers and TV channels scrambling to verify reports that claimed to know the identity of the dead assailant and also alleged that internet chat forums predicted the attack’s exact time and location, London’s proudly stoic residents attempted to travel home in the evening rush hour through a city, yet again, witnessing carnage.

On Wednesday night, the UK’s parliament building, that sits in the heart of the capital city, remained on shutdown as did a significant area around it.

“This was a day we’ve planned for but hoped would never happen. Sadly it is now a reality,” assistant commissioner Rowley added.