The jihadist claim came as armed police arrested eight people in raids over the attack on the symbol of Britain’s democracy that left three dead and sowed panic in the heart of London.

“An act of terrorism tried to silence our democracy,” a defiant Prime Minister Theresa 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,” May said.

IS said it was responsible, according to the Amaq propaganda agency linked to the group, the first time the jihadists have claimed 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.”

Britain is a member of the US-led coalition fighting IS in Iraq and Syria that the jihadist group has urged its followers to target.

Barely 24 hours later, 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.

The London carnage was unleashed the same day Brussels marked the first anniversary of bombings that killed 32 people and were also claimed by IS.

In the British parliament, defiant lawmakers returned to “business as usual” in the surreal silence of an area in the heart of London normally thronged with tourists.

Hundreds of extra police were on patrol 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 capital’s transport system in July 2005.

Wednesday’s attacker 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, stabbing to death a police officer before being shot dead.

The other victims were a 43-year-old mother and a man in his 50s.

Twenty-nine people were treated in hospital, including seven in critical condition, some with “catastrophic” injuries. Among them were French school children and foreign tourists.

Europe has been on high alert after a wave of deadly jihadist assaults over the past two years.

May said the attacker’s identity was known to police and the MI5 domestic intelligence service although she did not name him.

He was British-born and some years ago had been a “peripheral figure” investigated over concerns about violent extremism.

“There was no prior intelligence of his intent — or of the plot,” May said.

‘Acted alone’

Top anti-terror officer Mark Rowley said police arrested eight people in raids on six houses in London, the central city of Birmingham and elsewhere.

“It is still our belief… that this attacker acted alone yesterday and was inspired by international terrorism,” he said.

Press Association photos believed to be of the knifeman lying on a stretcher showed a burly man with black clothes and a beard.

Rowley acknowledged that Muslim communities “will feel anxious at this time” after previous extreme right-wing attacks and said police would work with community leaders to ensure protection.

The British flag over parliament flew at half-mast while forensic officers in white suits carried out a search of the courtyard outside where 48-year-old policeman Keith Palmer was slain.

“My thoughts, prayers, and deepest sympathy are with all those who have been affected by yesterday’s awful violence,” Queen Elizabeth II said, expressing her “enduring admiration” for the police.

London Mayor Sadiq Khan has called a candlelit vigil later Thursday. 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 vote to leave the EU.

That vote has triggered a push in Scotland for a second independence referendum but the parliament in Edinburgh this week postponed a debate and vote on the issue.

The debate is now due to resume Tuesday, the day before Britain is to trigger the Article 50 two-year EU divorce process.

Tourists among injured

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.

Several foreign tourists visiting one of London’s most iconic sights were also caught up in the violence.

French Foreign Minister Jean-Marc Ayrault travelled to London to visit three French pupils on a school trip who were among those hurt.

Three police officers were among those hurt and a seriously injured woman was rescued from the river after coming off the bridge.