Islamic State has claimed responsibility for Monday's blast at a concert by U.S. pop star Ariana Grande in Manchester, England that killed at least 22 people.
The group said that "a soldier of the caliphate planted bombs" then detonated them.
British police have said investigators believe the attacker was carrying an improvised explosive device, which he detonated, and that he died at the site. The blast has left at least 59 people wounded.
Prime Minister Theresa May said police and security officials believe they know the identity of the attacker, but are not yet ready to confirm it publicly.
The police department later said on Twitter that officers had arrested a 23-year-old man in South Manchester in connection with the attack, but did not give any information about how he was involved.
The blast happened in the lobby of the 21,000 seat Manchester Arena at the end of a concert by Grande.
Reaction from Grande
"Broken. From the bottom of my heart, I am so, so sorry," Grande wrote on Twitter after the blast. "I don't have words."
May said the blast was timed to "cause maximum carnage" and targeted "the young people of our society with cold calculation."
"All acts of terrorism are cowardly attacks on innocent people, but 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.
May and Jeremy Corbyn, the leader of the opposition Labour Party, agreed to suspend campaigning ahead of the country's June 8 elections.
US monitoring situation
The U.S. Department of Homeland Security said it was monitoring the situation in Manchester, and that it did not have any information showing a "specific credible threat" to music venues in the U.S.
President Donald Trump said the victims in Wednesday's attack were killed 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," Trump said. "I will call them from now on losers, because that's what they are."
He added, "We cannot stand a moment longer for the slaughter of innocent people."
WATCH: Trump reacts to Manchester attack
After the attack, Manchester police deployed hundreds of officers overnight and at one point conducted a precautionary controlled explosion near the arena of an object they later said turned out to not be anything suspicious.
Video from the concert showed thousands of concertgoers, many of them young girls, scrambling and screaming, trying to escape the building.
Some witnesses said the ground near the blast was covered with nuts and bolts.
Abandoned shoes, phones and jackets were scattered throughout the arena.
"It was a huge explosion. You could feel it in your chest. It was chaotic. Everybody was running and screaming just trying to get out," a concertgoer told Reuters.
In Photos: Manchester suicide bombing
Search for survivors, victims
Worried parents who had brought their children to the show crowded the streets outside the building. A nearby hotel opened its doors to the kids looking for their mothers and fathers.
Cab drivers turned off their meters and offered to drive people from the ill-fated concert to wherever they want to go.