- The Islamic State statement did not give any details about the attacker
- British PM Theresa May called the carnage a "callous, terrorist attack"
- Messages of support poured in from around the world, including from Trump
The Islamic State statement did not give any details about the attacker or how the blast was carried out late Monday. 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 defenseless 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 are 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. No one has yet claimed responsibility for the attack, which injured 59 others.
"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."
Messages of support poured in from around the world, including from President 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."
Charlotte Campbell told the BBC on Tuesday morning that she's "phoning everybody," including hospitals and centers trying to locate her 15-year-old daughter Olivia. She last spoke to her daughter on Monday night at the concert.
"She'd just seen the support act and said she was having an amazing time, and thanking me for letting her go," she said in an emotional interview.
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 .
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."
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."
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.
A father told the BBC that he was leaving the arena with his wife and daughter when the blast blew him through a set of doors. Afterward, the man, identified as Andy, said he saw about 30 people "scattered everywhere. Some of them looked dead."
Separated from his wife and daughter, he said, he "looked at some of the bodies trying to find my family."
He later found them, uninjured.
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.
Mark Harrison, who accompanied his 12-year-old daughter to the concert from Cumbria in northern England, said there weren't metal detectors or body checks at the arena's entrance, though bags were inspected and items such as water bottles had to be discarded.
"There was definitely a security presence, but anyone can come through the train station," said Harrison, 44.
The scenes of bloodied, panicked concertgoers running for safety brought to mind similar images at the Bataclan theater in Paris in November 2015.
The concert hall became the scene of extreme carnage after multiple gunmen burst in during a show by the American rock band Eagles of Death Metal and began shooting. The attack - for which the Islamic State later asserted responsibility - killed 89 people and injured hundreds more, becoming the deadliest event on French soil since World War II.
Britain has had fewer terrorist attacks in recent years than several of its European neighbors. Monday night's blast came two months after a speeding driver left four people dead on London's Westminster Bridge, then stabbed to death a police officer at the gates of Parliament.
Monday was the fourth anniversary of the killing of Lee Rigby, a British soldier who was attacked with a machete on the streets of southeast London. The two assailants, who were convicted of murder, said they were acting to avenge the killing of Muslims by British soldiers.
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 debate, although that may change when campaigning resumes.
(Adam reported from London. The Washington Post's Isaac Stanley-Becker in Manchester contributed to this report.)
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)