MANCHESTER, England (CNN) -- 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. An 8-year-old girl and an 18-year-old woman have been named as the first victims killed.
Police said they believed a man carrying explosives acted as a lone attacker and died in the blast, and that 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.
ISIS claimed responsibility for the attack but offered no evidence to support their claim.
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.
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.
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.
"Saffie was simply a beautiful little girl in every aspect of the word ... Saffie was quiet and unassuming with a creative flair," said Chris Upton, the head teacher at the Tarleton Community Primary School.
Georgina Callander, 18, was also killed, according to her school, the Bishop Rawstorne Church of England Academy.
"Georgina was a lovely young student who was very popular with her peers and the staff and always made the most of the opportunities she had at the school," a school statement said.
"All of our students will gather together today for a time of prayer and reflection and to give thanks for the life of Georgina."
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.
'Difficult dawn' 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 bomb attack has raised concerns that a more sophisticated network may exist in the country than previously thought.
Some 400 police were deployed in Manchester overnight, Greater Manchester Police Chief Constable Ian Hopkins said early Tuesday. A CNN journalist saw a heavy armed police presence in parts of the city, particularly outside the Royal Infirmary Hospital where several victims are being treated.
Greater Manchester Mayor Andy Burnham described the attack as "our darkest of nights."
"Manchester is today waking up to the most difficult of dawns," he said.
"These were children, young people and their families that those responsible chose to terrorize and kill. This was an evil act," he said.
London will also see a heavier police presence Tuesday, the mayor's office said.
Images from the scene show people running in a panic down stairs to escape the 21,000-seat arena amid the relentless screams of frightened children and young teenagers. Others showed bodies laying bloodied on the floor of the arena.
The explosion rocked the arena at around 10:30 p.m. (5:30 pm 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."
Coral Long, the mother of a 10-year-old concertgoer, told CNN they were getting ready to leave the arena when they heard a loud bang from the left side of the arena that sent the large crowd running. "How we weren't crushed to death is a miracle."
She said her daughter was devastated. "For her to be 10 years old and witness something like that is just horrific."
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.