British police on Thursday identified the man who killed three people in a rampage outside parliament as a 52-year-old with a string of convictions but none for terror offences.
The Islamic State (IS) militant group claimed the assailant behind the deadliest attack on British soil in 12 years was one of its "soldiers" acting on a call to target countries in the US-led coalition fighting the Islamist militants.
Police named him as English-born Khalid Masood and said he had convictions for assault including grievous bodily harm, possession of offensive weapons and public order offences ranging from 1983 to 2003.
However, it said Masood had never been convicted of any terrorism offences.
"(He) was not the subject of any investigations and there was no prior intelligence about his intent to mount a terrorist attack".
Some media said Masood, reportedly a married father-of-three, was believed to have worked as an English teacher and had given that as his occupation when he rented the car used in Wednesday's bloody rampage.
"He was a nice guy. I used to see him outside doing his garden," Iwona Romek, a former neighbour of his told the Birmingham Mail.
Masood was named after armed police arrested eight people in raids in London and Birmingham in central England, an area where he had lived.
A defiant Prime Minister Theresa May told parliament that Britain would not be cowed by the attack on the very symbol of the country's democracy.
"An act of terrorism tried to silence our democracy," May told a packed House of Commons, which stood for a minute's silence in remembrance of the victims.
"We are not afraid and our resolve will never waver in the face of terrorism."
Just hours after she spoke, Belgian police arrested a man as he tried to drive into a crowd at high speed in a shopping area in the port city of Antwerp.
That was the third incident in a week in Europe after London and a weekend attack at Paris's Orly airport, and came a day after the first anniversary of the Brussels suicide bombings that killed 32 people and were also claimed by IS.
Hundreds of extra police were on patrol in London as officers worked around the clock to piece together what happened in the deadliest attack in Britain since four suicide bombers killed 52 people on the city's transport system in July 2005.
The IS group said it was responsible, according to the Amaq propaganda agency, its first claim of an attack on British soil.
"The perpetrator of yesterday's attack in front of the British parliament was a soldier of the Islamic State and the operation was carried out in response to calls to target coalition countries," Amaq said citing a "security source."
The latest attack had echoes of the atrocities in Nice and Berlin when trucks ploughed into crowds of people, killing 86 people in the French Riviera city in July and 12 at a market in the German capital just days before Christmas.
As the British flag flew at half-mast, lawmakers returned to "business as usual" in the surreal silence of an area in the heart of London normally thronged with tourists.
The UN Security Council also observed a minute of silence, as Britain's allies vowed to stand by London in the fight against terrorism.
In his broad daylight attack, Masood mowed down pedestrians with a car along Westminster Bridge -- a busy traffic route and popular tourist spot with views of parliament and the Big Ben clock tower.
He then rammed the railings outside parliament and jumped out of the car, fatally stabbing 48-year-old unarmed police officer Keith Palmer before being shot dead.
The other victims were a 43-year-old British woman and an American in his 50s, while 29 people were treated in hospital including French school children and foreign tourists.
May said the attacker had been a "peripheral figure" investigated over concerns about violent extremism.
Pictures believed to be of Masood lying on a stretcher being treated by medics showed a burly man with black clothes and a beard.
"It is still our belief... that this attacker acted alone yesterday and was inspired by international terrorism," Britain's top anti-terror officer Mark Rowley said.
In Birmingham, home to large South Asian and Muslim communities, Muhammad Afzal, the head of the central mosque, said the attacker's motivations had nothing to do with true Islam.
Rowley acknowledged that Muslim communities will "feel anxious" over fears of an Islamophobic backlash, and said police would work with community leaders to ensure protection.
Britain's last terror attack was the 2016 assassination of MP Jo Cox by a pro-Nazi sympathiser shortly before the historic but deeply divisive June vote to leave the EU.
Scotland, where there is a fresh push for independence following Brexit, delayed a parliamentary debate on a new referendum until Tuesday, the day before Britain is to trigger the Article 50 two-year EU divorce process.
A Spanish diplomatic source confirmed that one of the dead was 43-year-old British citizen Aysha Frade, whose mother was Spanish.
Media reports said she was on her way to pick up her two daughters, aged seven and nine, from school.
Kurt Cochran from the United States was named as the third victim by President Donald Trump, who called him a "great American".