Four people were killed in the Westminster terror attack, police have confirmed.
The number includes the police officer who was stabbed and his attacker, Scotland Yard's top anti-terror officer Mark Rowley said.
The other two victims are understood to have died on the bridge, he added.
He also said at least 20 people had been injured.
Mr Rowley also said police believed there to have been only one suspect, although he said they were carrying out thorough checks.
The attacker, armed with two large knives, mowed down pedestrians with his car on Westminster Bridge, including schoolchildren, then rushed at the gates in front of the Houses of Parliament, stabbing a policeman before he was shot by armed officers.
Eyewitnesses described scenes of terror when gunfire rang out as the attacker approached a second officer within yards of the Houses of Parliament.
Mr Rowley told reporters outside New Scotland Yard: "This was a day we've planned for but hoped would never happen. Sadly it is now a reality."
Paramedics fought to save the officer's life and that of his attacker on the floor of the cobbled courtyard in front of Parliament, with Foreign Office minister Tobias Ellwood among those who rushed to help.
The police officer was wheeled away on a stretcher with his face covered.
Mr Ellwood, who lost his brother in the Bali bombing, could be seen pumping the officer's chest then standing above him, his hands and face smeared with blood.
Other armed officers, some in plain clothes and wearing balaclavas, swarmed around the yard just feet from where MPs had earlier attended Prime Minister's Questions.
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