The Islamic State on Tuesday one of its "soldiers" carried out a deadly blast in Manchester that killed at least 22 people, mainly teenagers and others streaming out of a pop concert.
Another 59 people, including 12 under the age of 16, were injured in the explosion, which happened in the foyer of Manchester Arena on Monday night, as crowds began leaving the concert by US pop star Ariana Grande.
The Islamic State statement did not give any details about the attacker or how the blast was carried out. The statement was posted on the online messaging service Telegram and later noted by the SITE Intelligence Group, which monitors militant websites.
The Islamic State often quickly proclaims links to attacks, but some previous claims have not been proven.
British Prime Minister Theresa May called the carnage a "callous terrorist attack".
"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," she said, speaking outside of Downing Street, where flags were flying at half mast.
She called it among the worst terrorist incidents in Britain and "the worst ever to hit the north of England".
Authorities believe they know the identity of the assailant, she added, "but at this stage of their investigations, we cannot confirm his name".
In a statement, the Greater Manchester Police said that they arrested a 23-year-old man in south Manchester in connection with the attack as hundreds of police swarmed through the city in the aftermath of the blast.
Authorities are trying to determine if the suicide bomber acted alone or was part of a larger network.
"We believe at this stage the attack last night was conducted by one man," said Greater Manchester Police Chief Constable Ian Hopkins at a televised news conference. "We believe the attacker was carrying an improvised explosive device, which he detonated, causing this atrocity."
Witnesses - many of them teenage girls - said they heard a huge bang coming from outside the auditorium.
Some mistook the bang of the bomb for exploding balloons, which had been released at the end of the concert.
The injured are being treated at eight hospitals in Greater Manchester. Many have life-threatening conditions, the prime minister said.
Messages of support poured in from around the world, including from US President Donald Trump.
"We stand in absolute solidarity with the people of the United Kingdom," he said at a news conference in Bethlehem, and called those responsible "evil losers in life".
In France, the scene of several terrorist attacks over the past year, French Prime Minister Edouard Philippe called on people to be vigilant in the face of "a threat which is more present than ever before".
Other world leaders too expressed sympathy and condemned the attack. In India, Congress leader Sonia Gandhi was among the first to express solidarity with the victims, even as Prime Minister Narendra Modi was yet to react till the time of writing on Tuesday.
The bombing appeared intended to inflict the maximum possible damage on young concertgoers - many of them in their early teens - who were making their way out of the Manchester Arena. Police said the blast occurred about 10:30 pm local time, minutes after Ariana Grande had finished her set.
The explosion set off a panicked reaction as fans struggled to flee and parents and teens searched for one another amid the carnage. Well into Tuesday morning, fathers and mothers who had lost contact with their children posted desperate pleas for information on social media using the hashtag #ManchesterMissing.
Andy Burnham, the mayor of Greater Manchester, called it an "evil act" but praised the "spirit of Manchester that will prevail and hold us together."
He said that Manchester is "grieving today, but we are strong."
It is the worst terrorist strike on British soil since 2005, when Islamist extremists bombed the London subway and a bus, killing 54 people.
Britain has been on high alert for a major attack for several years, with authorities saying that a mass-casualty attack was likely.
Grande, who is wildly popular both in Britain and the United States, was not injured in the attack. She expressed her sorrow in a tweet hours after the explosion, saying she was "broken. from the bottom of my heart, i am so sorry. i don't have words."
Cellphone video showed chaotic scenes of people screaming and running in the aftermath of the blast. Concertgoers said that they saw nuts and bolts littering the ground near the blast scene and that the smell of explosives hung in the air.
The local hospital, Wythenshawe, said it was dealing with "mass casualties." Eight other hospitals across the region were activated to treat the injured, and emergency supplies of blood were rushed in.
Fans of Grande had come from across northern England to see the concert. On Twitter, people offered a place to stay for those stranded in the city, using the hashtag #RoomForManchester.
Karen Ford, a witness, told the BBC that "there were kids outside, crying on the phone, trying to find their parents."
The arena is one of the largest indoor venues in Europe and has a capacity of 21,000. The attack took place near one of the exits of the facility, in a public space.
Jake Taylor, a former security guard at the arena, said its layout makes absolute safety impossible, as it is housed in the bustling Victoria train station.
"You can't stop people from getting through the train station," said Taylor, 26.
Monday's blast comes with just over two weeks to go before Britain holds a national election. Campaigning was suspended Tuesday, and perhaps beyond. Security has not featured as a prominent part of the election debate, although that may change when campaigning resumes.