British Prime Minister Theresa May said it was "now beyond doubt" that it was 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."
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 p.m., minutes after pop star 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.
"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 .
The U.S. Department of Homeland Security said late Monday that there was "no information to indicate a specific credible threat involving music venues in the United States," but added that Americans may see "increased security in and around public places and events as officials take additional precautions."
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."
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.
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. Manchester transport police said the explosion occurred in the arena's foyer, where people were congregating to buy concert merchandise. Manchester Arena said the attack took place just outside the facility, in a public space.
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.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)