The dead police officer was one of the armed guards at parliament, Mark Rowley said, adding that other victims were hurt in the car-ramming attack on Westminster Bridge.
At least 40 people were wounded in the attack, which forced Britain's parliament into lock-down.
The assailant has not been identified. Rowley said police think they know the identity of the man but would not reveal details. He said Islamic extremism is suspected in the attack.
He said extra armed police would be on the streets in the coming days to reassure the public, and hundreds of police officers are working on the case.
He identified the police officer who died as Keith Palmer, 48.
Prime Minister Theresa May condemned the "sick and depraved terrorist attack" in London and said the targeting Parliament was no accident.
In a late-night statement outside her Downing Street office Wednesday, a defiant May said the nation will not give in to terror and those who targeted the seat of power in Britain.
She insisted that "tomorrow morning Parliament will meet as normal," and urged the country to move on and behave as normal on Thursday.
May said "we will all move forward together, never giving in to terror and never allowing the voices of hate and evil to drive us apart."
An injured woman was recovered alive from the River Thames, the Port of London Authority said.
"A female member of the public was retrieved from the river near Westminster Bridge, with injuries, and is receiving medical treatment," a spokesman said.
Three French students on a school trip were among the injured, confirmed France's foreign ministry spokesman, Romain Nadal.
France Bleu said around a dozen of the school's students were on the bridge at the time of the attack, and the three injured were taken to hospital.
Witnesses said they saw a car plough into people walking on Westminster Bridge - a busy traffic route that is also a popular tourist spot with its views of parliament and the famed clock tower which is home to the Big Ben bell.
The vehicle then veered towards the Palace of Westminster, home to both houses of parliament and parliamentarians' offices, hitting the railings outside. A man then jumped out of the car, attempted to enter the building's grounds, and stabbing a police officer. He was shot dead at close range.
French Prime Minister Bernard Cazeneuve tweeted: "Solidarity with our British friends, horribly attacked, full support for the injured French students, their families and their friends."
The White House also condemned the attack.
"We applaud the quick response that the British police and their first responders made to the situation for the victims in this. They are in our thoughts and our prayers," said Press Secretary Sean Spicer.
German Chancellor Angela Merkel said her country stood by Britain "firmly and resolutely".
"Although the background to these acts are not yet clear, I reaffirm that Germany and its citizens stand firmly and resolutely alongside Britons in the struggle against all forms of terrorism," Merkel said in a statement.
The incident came as Belgium marked the first anniversary of the Islamic State group's suicide attacks in Brussels, which killed 32 people last year.
No link has been drawn by any official, and there has been no claim of responsibility yet made on any of IS' usual media outlets.
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