Police have named a Manchester-born son of Libyan immigrants as the suicide bomber responsible for the terror attack at an Ariana Grande concert that left at least 22 people dead.

The man, Salman Abedi, lived less than six kilometres from Manchester Arena, where he detonated a home made explosive shortly after 10.30pm local time as crowds passed through the foyer at the end of the concert.

Greater Manchester Police Chief Constable Ian Hopkins identified the bomber at a media conference Wednesday morning (AEST), but declined to provide any further details saying a coroner was yet to officially identified him.

British Prime Minister Theresa May announced Wednesday morning that the UK’s terror alert level had been raised from severe to critical indicating a further attack is “expected imminently”.

Hundreds of military personnel will now be deployed to protect key sites around the UK.

It is just the third time Britain has reached a critical alert level. The first time was in 2006 when a plot to blow up transatlantic airliners with liquid bombs was foiled, followed in 2007 when a man tried to bomb a London nightclub, before going on to attack Glasgow Airport.

Eight-year-old Saffie Rose Roussos was among those killed by the bombing, which also left 59 people injured – 12 of whom were under the age of 16.

Other victims identified so far were 18-year-old Georgina Callander and 28-year-old John Atkinson.

Eight-year-old Saffie Rose Roussos was among the bombing victims. Photo: Twitter

Several people, including five teenagers, are thought to be missing after the attack, the worst on British soil since the 2005 suicide bombings.

While it is believed Abedi was the lone bomber, police have searched several Manchester properties and arrested a 23-year-old man as they seek any associates who may have assisted in building the bomb and planning the attack.

“Our priority, along with the police counter terrorism network and our security partners, is to continue to establish whether he was acting alone or working as part of a wider network,” Chief Constable Hopkins said.

The arrested man has been identified by local media as Abedi’s brother.

Chief Constable Hopkins said police had also raided three homes in Manchester – including what was reported as Abedi’s family home in the suburb of Fallowfield, where a controlled explosion was conducted to “enable safe entry”.

Investigators are also trawling through hours of CCTV footage and urgently appealing for any available dashcam footage from the area leading up to the bombing as they work to piece together the bomber’s movements and eliminate the possibility of further attacks.

Islamic State has taken credit for the slaughter, hailing the Abedi as a hero and “soldier of the Caliphate”.

Abedi reportedly attended school in Manchester and then Salford University before dropping out.

Friends told the BBC they remembered him as a telented footballer, a keen supporter of Manchester United and a user of cannabis.

Abedi is the second for four children and his parents are thought to have returned to live in Libya in recent years.

US and British media say Abedi had been in Libya before returning to the UK just days before the bombing.

CBS reported that Abedi was known to British authorities prior to Monday night’s attack, though no official confirmation has so far been forthcoming.

Prime Minister May said Abedi deliberately chose “his time and place to cause maximum carnage” in the young crowd.

“All acts of terrorism are cowardly,” Ms May said outside her Downing Street office after a meeting with British security and intelligence chiefs.

“But this attack stands out for its appalling sickening cowardice, deliberately targeting innocent, defenceless children and young people who should have been enjoying one of the most memorable nights of their lives.”

As thousands gathered for a vigil  in Manchester’s Albert Square to mark the tragedy, hundreds of others have visited the city’s various hospitals with gifts and food for the injured, and to offer donations of blood.

Thousands attended a vigil in central Manchester less than 24 hours after the attack. Photo: Getty

Evil losers

US President Donald Trump described the perpetrator of the Manchester attack and other terrorists as “evil losers”.

“I won’t call them monsters because they would like that term. I will call them losers,” President Trump said from Israel before leaving for Italy on his first presidential trip.

The Queen expressed her “deepest sympathy to all who have been affected by this dreadful event” and praised the heroism of emergency workers who tended to the dead and dying with disregard for their own safety.

“I want to thank all the members of the emergency services, who have responded with such professionalism and care,” she said.