The escalated terror threat came as vigils were held in Manchester and elsewhere across the country Tuesday night.The British Consulate in Hong Kong has opened a condolence book for the public to sign.

Watch: video captures chaos at Manchester concert

UK police said soldiers would initially be on guard duties at fixed points and could also be sent to patrol transport hubs, concerts and sports events.

The plan, which has never before been used and is codenamed Operation Temperer, was first revealed after the November 2015 Paris terror attacks and is believed to allow up to 5,000 troops to be deployed.

British media said Abedi was born in Manchester, northwest England, and that his Libyan parents had fled the regime of dictator Muammar Gaddafi.

Police staged an armed raid on a Manchester address believed to be where Abedi lived, carrying out a controlled explosion to gain entry after arresting a 23-year-old man earlier Tuesday as part of the investigation.

Abedi was reported to be a former business student who dropped out of university and turned to radical Islam.

Watch: police chief confirms suspect in blast

Monday’s attack came just over two weeks before Britain votes in a general election and campaigning was suspended by all the main political parties.

Threatening more attacks, IS said in a statement published on its social media channels: “One of the caliphate’s soldiers placed bombs among the crowds.”

The suicide bombing came at the conclusion of Grande’s concert at the 21,000-capacity Manchester Arena, one of Europe’s largest indoor venues.

Witnesses described the horror when the bomber blew himself up outside one of the exits as the mostly youthful audience was leaving, with many parents waiting outside to collect their children.

“When we left, down the stairs there was probably early teenagers lying on the floor covered in blood and blood on the walls where they’d been laid, so it was just horrifying,” female concert-goer Alex Grayson said.

Terrified fans, many of them teenage girls, fled the arena in panic. Grande, a 23-year-old former child television star, described herself as “broken” by the attack.

Some 59 people were taken to hospital, many with life-threatening conditions. Twelve of them were aged under 16.

Witnesses reported seeing bodies on the floor after the blast on Monday night, and some fans were trampled as panicked crowds tried to flee the venue.

Families were separated, with dozens of young people taken to nearby hotels overnight, and some parents were still desperately searching for their children on Tuesday.

“I’m just hearing nothing - her phone’s dead,” Charlotte Campbell, whose 15-year-old daughter Olivia was at the concert, told BBC radio.

In the city centre, thousands gathered for a multi-faith vigil outside the Town Hall to remember the victims, with defiant chants of “Manchester! Manchester!” ringing around.

Poet Tony Walsh recalled the city’s history in a poem entitled “This Is The Place”, which drew huge cheers as he recounted the gritty attitude that made the city a heartland of industrial and cultural creativity.

In a poignant moment he referred to the “Mancunian way to survive” and said: “In the face of a challenge we always stand tall.”

The attack was the deadliest in Britain since July 7, 2005 when four suicide bombers inspired by Al-Qaeda attacked London’s transport system during rush hour, killing 52 people and wounding 700 more.

It revived memories of the November 2015 attack at the Bataclan concert hall in Paris in which armed men wearing explosive belts stormed in and killed 90 people.

That attack was also claimed by IS, as was one in March by a knifeman at the gates of the British parliament - although police downplayed that claim.

Queen Elizabeth II condemned the Manchester attack as an “act of barbarity” and observed a minute’s silence at a Buckingham Palace garden reception.

In a city famed globally for its football teams and pop bands like Oasis, The Stone Roses, The Smiths and Take That, showbusiness stars and teams expressed their horror at the carnage.

“Words don’t really do justice for how we all feel. We’re numb,” Manchester United’s executive vice-chairman Ed Woodward said, ahead of Wednesday’s Europa League final against Amsterdam giants Ajax.

The players will wear black armbands for the game in Stockholm and a minute’s silence will be held.

Britain’s international terror threat level was last at the critical level in June 2007.

May, who is due to contest a national election on June 8, said the man named by police as the attacker, Salman Abedi, was born and brought up in Britain and investigations into whether he was working alone were ongoing.

“The work undertaken throughout the day has revealed that it is a possibility we cannot ignore that there is a wider group of individuals linked to this attack,” she said.