Counter-terror detectives will continue searching for clues about how an armed attacker brought death and destruction to the streets of London, killing a police officer, three members of the public, and injuring dozens more.
Police last night named 48-year-old Pc Keith Palmer as the unarmed police officer killed in Westminster.
The attacker, armed with two large knives, mowed down pedestrians with a car on Westminster Bridge, including schoolchildren, then rushed at the gates in front of the Houses of Parliament, stabbing Mr Palmer before being shot dead by other officers.
Speaking outside Scotland Yard on Wednesday night, acting deputy commissioner Mark Rowley, the Met's senior anti-terror officer, said about 40 people had been injured, several - including two police officers - seriously.
He declined to name the attacker, but said police suspect he was "inspired by international terrorism" - and that they believed they knew who he was.
Paying tribute to Pc Palmer, a member of the Parliamentary and Diplomatic Protection Squad who had served the police for 15 years, Mr Rowley said: "Today in Westminster we saw tragic events unfold and our thoughts are with those who lost loved ones, those who were injured and all those affected by this attack.
"One of those who died today was a police officer, Pc Keith Palmer, a member of our parliamentary and diplomatic protection command. Keith, aged 48, had 15 years' service and was a husband and father.
"He was someone who left for work today expecting to return home at the end of his shift, and he had every right to expect that would happen."
It came as the Prime Minister vowed to defeat what she called "the forces of evil".
Theresa May, who was in Parliament at the time of the attack, praised the bravery of police officers who killed the attacker as he sought another victim.
She said any attempt to defeat the values that Parliament stood for was "doomed to failure".
Parliament will reopen on Thursday morning and parts of the crime scene will restrict some of the entrances, with emphasis on business returning to normal as quickly as possible.
But the attack has delayed the Queen's planned visit to the new headquarters of the Metropolitan Police.
A Buckingham Palace spokesman said the decision had been taken to postpone the Queen's engagement in light of the attack.
Stories of heroism and bravery emerged from the incident, which brought central London to a standstill and closed transport networks around the capital shortly after 2.30pm on Wednesday.
Paramedics fought to save Mr Palmer'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.
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.
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.
The knifeman drove a grey Hyundai i40 across Westminster Bridge before crashing it into railings, then running through the gates of the Palace of Westminster.
His attack left a trail of destruction as paramedics tended to victims on the bridge and at the gate.
One woman hit by the attacker's car before he reached Parliament was confirmed dead by a doctor at St Thomas' Hospital. She said others on the bridge suffered "catastrophic injuries".
Another woman who fell into the Thames was rescued and given urgent medical treatment on a nearby pier.
A party of French schoolchildren were among those targeted on the bridge, with three injured.
Donald Trump was among world leaders quick to offer their support.
The White House said the US president spoke to Mrs May in a phone call following the bloody events in London.