MANCHESTER, England -- At least 22 people, including children, have been killed in a blast at an Ariana Grande concert in Manchester, in the deadliest terror attack on British soil since the 2005 London bombings.
A powerful explosion shook part of the cavernous Manchester Arena late Monday as concertgoers streamed out following the American pop star's last song.
ISIS claimed responsibility for the attack but offered no evidence to support its claim.
An 8-year-old girl and an 18-year-old woman have been named as the first victims of the attack, which has drawn condemnation and horror from around the world as a heinous assault targeting children.
Police believe a man carrying explosives acted as a lone attacker and died in the blast, and they are now trying to determine if he may have been part of a wider network. A 23-year-old man was arrested Tuesday in south Manchester in connection with the bombing.
Police have carried out two raids in Manchester. Queen Elizabeth II described bombing as 'act of barbarity'. 59 people injured, some in life-threatening situations. US President Donald Trump slams attackers as 'losers'. Ariana Grande tweets that the incident has left her 'broken'. British Prime Minister says police have identified the suspect.
ISIS said on its Telegram channel Tuesday that a "soldier of the caliphate" was able to "plant explosive devices" at the arena, a US counter-terrorism source told CNN.
But police have not said who was responsible for the bombing and ISIS routinely claims attacks it has no proven links with.
Video from inside the arena showed girls screaming as they scrambled over chairs and railings to escape the 21,000-seat venue, while photographs showed bodies laying bloodied on the floor.
Speaking after an emergency Cabinet meeting, British Prime Minister Theresa May condemned the attack as "callous" and "cowardly."
May said police had identified the suspected attacker, though she did not reveal his name.
"We struggle to comprehend the warped and twisted mind that sees a room packed with young children not a scene to cherish but as an opportunity for carnage," May said in London, before leaving for Manchester.
Young victims named
Eight-year-old Saffie Rose Roussos from Leyland has been named as one of the victims killed, the Lancashire County Council confirmed.
Chris Upton, the head teacher at the Tarleton Community Primary School, described her as "simply a beautiful little girl" who was "quiet and unassuming with a creative flair."
Georgina Callander, 18, was also killed, according to her school, the Bishop Rawstorne Church of England Academy.
"All of our students will gather together today for a time of prayer and reflection and to give thanks for the life of Georgina," the school said.
At least 12 victims aged 16 or under are being treated at a children's hospital for serious injuries, some of them fighting for their lives, a Manchester health official said.
Grande, who had just finished the first of three scheduled UK performances, tweeted about her devastation several hours later: "broken. from the bottom of my heart, i am so so sorry. i don't have words."
Queen Elizabeth II said Tuesday that "the whole nation has been shocked."
"I would like to express my admiration for the way the people of Manchester have responded, with humanity and compassion, to this act of barbarity," she said.
'Darkest of nights' in Manchester
Britain has been under a "severe" terror threat alert for three years and there has been an uptick in terror-related arrests in recent months.
Police have been warning that another attack was highly likely after a man plowed his car into a crowd on London's Westminister Bridge in March and stabbed a policeman, in an attack that left six dead.
Monday's bombing has raised concerns that a more sophisticated network may exist in the country than previously thought.
Greater Manchester Mayor Andy Burnham described the attack as "our darkest of nights."
"These were children, young people and their families that those responsible chose to terrorize and kill. This was an evil act," he said.
Around 400 police were deployed overnight following the attack, and on Tuesday, large groups of armed police were seen at several parts of the city. Security has also been boosted in London.
People began paying their respects to the victims on Tuesday afternoon. A family of four arrived at St Ann's Square in the city center with a huge bouquet of flowers and colorful balloons to offer.
"I'm just feeling really down for all the families that lost their children and family members," said Michael Heveril.
"It's quite close to home -- I never thought anything like this would ever happen in Manchester."
The explosion rocked the arena at around 10:30 p.m. (5:30 p.m. ET), and the sound of wailing sirens cut through the smoky aftermath of the blast soon after.
Crying children and parents desperately tried to find each other as cell phone signals faltered in the deluge of calls, witnesses said.
Manchester resident Charlotte Campbell told CNN as she was still waiting for news on her 15-year-old daughter, Olivia. "We've tried everything we can. They're telling us to wait by the phones," she said.
Olivia had gone to the concert with a friend and neither have been in contact. Her father is now looking for the girls.
"I want her home and I want her safe. ... I just want her to walk through the door."
Trump calls attackers 'losers'
US President Donald Trump slammed the attack, saying that terrorists were "losers."
"So many young beautiful innocent people living and enjoying their lives murdered by evil losers in life. I won't call them monsters because they would like that term. They would think that's a great name. I will call them from now on losers because that's what they are," he said.
"This wicked ideology must be obliterated."
A US Department of Homeland Security statement said it was "closely monitoring" the situation.
CNN's Samantha Beech, Darran Simon, Lauren Said-Moorhouse, Bryony Jones, Pamela Brown, Evan Perez and Ross Levitt contributed to this report.