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 each other 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.
British Prime Minister Theresa May issued a statement in the early hours of Tuesday saying that authorities were "working to establish the full details of what is being treated by the police as an appalling terrorist attack."
Greater Manchester Police said the blast was being "treated as a terrorist incident until police know otherwise."
There was no immediate claim of responsibility for the blast, and police did not speculate about possible motives.
If confirmed as a terrorist attack, it would be the worst 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."
Britain has been on high alert for a major attack for several years, with authorities saying that a mass-casualty attack was likely.
Manchester police said they were working closely with national authorities to determine the cause of the explosion. Among the priorities for investigators will be to figure out whether it was part of a broader plot.
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. The arena was packed with attendees and pink balloons that had fallen from the ceiling during the final song. Initially, concertgoers said they thought popping balloons had set off a panic, or that the screams were those of fans who had caught a glimpse of Grande.
But witnesses later reported seeing the prone bodies of those who had been wounded and killed, as well as others who were streaked with blood and were staggering away from the scene. Some were injured in the rush to get out, with people being trampled as thousands sought to escape.
In video of the moment that the explosion detonated, a concussive boom breaks through the chatter of fans heading for the exits. "Oh my god, what just happened?" a female voice can be heard asking. "What's going on?"
Later video showed people diving over railings. 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." Five other hospitals across the city were activated to treat the injured, and emergency supplies of blood were rushed in.
Heavily armed police and emergency services swarmed the arena, with ambulances — their blue lights flashing — rushing to the scene. The local emergency-
response service advised the public to call only "for life-threatening emergencies."
Many of those attending the concert were teenagers going to their first concert. Witnesses reported that outside the arena, parents were frantically attempting to locate their children. Many parents and teens later gathered at a nearby Holiday Inn that was established as a meeting point.
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.
Parents posted pictures of missing children on social media, pleading for information. Police set up a hotline for those looking to connect with missing relatives.
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.
Other witnesses described a loud bang, followed by terrified shouts. "It was really scary," Michelle Sullivan, who was attending the concert with her 12- and 15-year-old daughters, told the BBC. "Just as the lights have gone down, we heard a really loud explosion. . . . Everybody screamed."
"When we got out, they just said, 'Keep on running, keep on running.' "
Karen Ford, a witness, told the BBC that "there were kids outside, crying on the phone, trying to find their parents."
About 1:30 a.m., police announced that there would be a controlled explosion after a suspicious object was found. A loud bang was heard minutes later. Police later said the item that had been found was discarded clothing, not an explosive device.
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.
Although nobody immediately asserted responsibility for Monday's violence, 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.
Grande is a 23-year-old pop singer and actress who has been in the public spotlight since 2010, when she began appearing on the Nickelodeon television show "Victorious." More recently, the former teen idol has been touring to promote her third studio album, "Dangerous Woman." She has sold more than 1.7 million albums in recent years.
The singer has more than 45 million followers on Twitter. Grande is also one of the most popular people on Instagram, with 105 million followers — more than even Beyoncé, Taylor Swift or Kim Kardashian. She was scheduled to play two shows in London later this week before traveling to Belgium, according to her tour dates.
Holley reported from Washington. Devlin Barrett in Washington contributed to this report.