The Met Police have named their colleague killed in yesterday's attack on Westminster as Pc Keith Palmer.
The unarmed police officer was among five people killed in a suspected terror incident.
The 48-year-old husband and father was stabbed by a man armed with two large knives inside the grounds of the Houses of Parliament.
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Minutes earlier the attacker had mown down pedestrians with a car on Westminster Bridge, including schoolchildren.
Speaking outside Scotland Yard, acting deputy commissioner Mark Rowley, the Met's top 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 believe he was "inspired by international terrorism".
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.
"I can also now confirm that there are three members of the public who have lost their lives in the attack.
"Specially-trained family liaison officers have been deployed to support them.
"And, as I confirmed earlier, the suspected attacker was shot dead by an armed officer.
"Therefore, meaning that now in total we have five people who died today."
Mr Rowley said three police officers were among those injured, two of whom were in a serious condition.
He refused to comment on the identity of the attacker, but said: "Our working assumption is that he was inspired by international terrorism."
Explaining why Pc Palmer was unarmed, Mr Rowley said: "Our parliamentary protection team are a combination of armed and unarmed officers doing different roles, and sadly the officer who lost his life today was unarmed.
"He was supported by armed colleagues who shot and killed the attacker.
"He (the attacker) tried to enter Parliament and was stopped at the gate."
Mr Rowley said there was a "range of nationalities" among the injured, but refused to give further information about where they were from.
He praised the efforts of those involved, saying while it was an "incredibly sad and sombre day", he felt "pride in the swift and brave response from our officers, especially those who had to confront the terrorist".
Hundreds of officers will work throughout the night on the investigation, interviewing witnesses, scouring CCTV and focusing on the suspect's "motivation, preparation and his associates".
Mr Rowley said: "We are forensically examining a complicated crime scene that covers a wide area, and as with all investigations of this nature it will take us some time to work through the painstaking work necessary to get all of the relevant evidence."
The lockdown on Parliament, which saw hundreds of MPs and parliamentary staff kept inside Commons buildings for their safety, has now been lifted.
Parliament will reopen on Thursday and parts of the crime scene will restrict some of the entrances, but business must return to normal as quickly as possible.
Mr Rowley said that while the threat level of "severe" will remain, "on a precautionary basis across the country we are stepping up police patrols - unarmed and armed - and the public will see far more of their local police forces over the forthcoming days, particularly in crowded places and iconic locations".
He said: "Terrorists have a clear aim, and that is to create discord, distrust and to create fear.
"The police stand together with all communities in the UK and will take action against anyone who seeks to undermine society, especially where crimes are motivated by hate.
"We know that our Muslim communities will feel anxious at this time, given the past behaviour of the extreme right wing, and we will continue to work with all community leaders in the coming days."
"Heartbroken" former colleague of Pc Palmer's, Conservative MP James Cleverly, paid tribute to the "lovely man" he had known for a quarter of a century.
He wrote of the 48-year-old father and husband: "I've known Keith for 25 years. We served together in the Royal Artillery before he became a copper.
"A lovely man, a friend. I'm heartbroken.
"My thoughts are with the family, friends and colleagues of Pc Keith Palmer. A brave man."
Scotland Yard's top anti-terror officer Mark Rowley said of his colleague: "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."
The Metropolitan Police Federation, which represents officers from the force, said: "PC Keith Palmer. Our brave Met #Police colleague who today made the ultimate sacrifice #LestWeForget"
A chorus of thanks also came from politicians on all sides of the House.
Home Affairs Select Committee chairwoman Yvette Cooper, who was in the Palace of Westminster when the terrorist burst through the gates, said: "Thank you Pc Keith Palmer - to your bravery many others owe their lives and safety"
Newcastle Central's MP Chi Onwurah tweeted: "As I left Parliament a police officer apologised for keeping us waiting. Cannot overstate courage & grace of those who protect us Thank you."
One woman who said she had her life saved by Pc Palmer when she worked as a police officer paid tribute to the "absolutely lovely man".