Police are hunting for what could be a second suspect on the loose after the Westminster terror attack.

A Metropolitan Police spokesman said it was thought there had only been one person involved.

But he added the building would remain on lockdown while officers looked for any other attackers as a precaution.

Four people have died, including a police officer, a man believed to be the attacker and a woman, with 20 others injured in incidents at Parliament and on Westminster Bridge.

A car, thought to have been driven by the attacker, ploughed into a crowd of pedestrians, before crashing into railings.

An officer guarding the building was then stabbed and armed officers opened fire on the apparent assailant.

The police spokesman said he hoped people would understand why Parliament remained on lockdown as officers searched the building.

"Whilst we currently believe there was only one attacker, I'm sure the public will understand us taking every precaution in locking down and searching the area as thoroughly and exhaustively as possible," he said.

"It would foolish to be over-confident so early on."

The Independent's Joe Watts tweeted that he and other journalists had been "herded into room on the estate for safety".

"Odd atmosphere in here. Some people working, others looking shaken others angry at being herded around the place," he added.

Hundreds of MPs and others were gathered in Westminster Hall within Parliament, awaiting evacuation. Cabinet Ministers were seen accompanied by bodyguards.