The London policeman who challenged a terrorist at the House of Commons and was stabbed to death, was unarmed.
PC Keith Palmer was among four people killed before the terrorist was shot dead by other policemen.
The 48-year-old husband and father was stabbed with two large knives inside the grounds of the Houses of Parliament.
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:
"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.
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."
TRIBUTES TO FALLEN POLICEMAN
The bravery of murdered PC Keith Palmer was hailed by colleagues, friends and the many MPs whom he was charged with protecting.
"Heartbroken" former colleague, 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."
Greater Manchester Police chief constable Ian Hopkins added: "#RIP PC Keith Palmer, brutally murdered today carrying out his duty helping to protect Parliament and the freedom of our Country."
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"
Labour MP Barry Gardiner was also caught up in the lockdown and posted a picture of armed police moving through Parliament.
He tweeted: "Going to bed alive, safe and well thanks to Pc Keith Palmer. God bless him, his family and all those killed or injured. Terror will not win."
Geoffrey Cox, Conservative MP, wrote: "Deeply sad that a brave Pc, Keith Palmer, died in the service of his country, doing his duty today. We should be proud to know such men."
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."
I owe him everything. He was a wonderful dad, and wonderful husband to his wife and everything
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".
Giving her name only as Nina, from Orpington, south-east London, she told radio station LBC: "He actually did save my life in a car accident on duty, I was a police officer.
"I was a police officer at the time, in 2007.
"I had a car accident on duty, I was the passenger. I was the passenger and he was one of the police officers who actually saved my life.
"I owe him everything. He was a wonderful dad, and wonderful husband to his wife and everything."
Reflecting on the officer's attitude to his work, she said: "He didn't just do his job to go and get paid. He actually cared about the people.
"He really did, he was a lovely man, absolutely lovely man, and everyone's going to really miss him."