MANCHESTER, England — An explosion that may have been a suicide bombing killed at least 19 people on Monday night and wounded about 50 others at an Ariana Grande concert filled with adoring adolescent fans, in what the police were treating as a terrorist attack.
Panic and mayhem seized the crowd at the Manchester Arena as the blast reverberated through the building, just as the show was ending and pink balloons were dropping from the rafters in a signature flourish by Ms. Grande, a 23-year-old pop star on an international tour.
Traumatized concertgoers, including children separated from parents, screamed and fled in what appeared to be the deadliest episode of terrorism in Britain since the 2005 London subway bombings.
Prime Minister Theresa May said her thoughts were with the victims and their families in “what is being treated by the police as an appalling terrorist attack.”
There was no immediate word from the police on the precise cause of the blast, but unconfirmed reports said a suicide bomber might have detonated a nail-filled explosive device.
Intelligence officials in the United States were briefed on the Manchester explosion late Monday and were told it appeared to be a terrorist attack, according to a senior official who was not authorized to discuss the matter publicly and spoke on the condition of anonymity.
The scene immediately evoked the terrorist attacks in Paris in November 2015, which included a deadly assault inside the Bataclan concert hall, where the Eagles of Death Metal had been playing. But unlike the Bataclan, the Manchester concert was filled with young teenagers.
“This is currently being treated as a terrorist incident until the police know otherwise,” the Manchester police said in a Twitter post.
People at the concert said they had heard what sounded like explosions at the end of the show, around 10:30 p.m.
At least one explosion went off in the foyer of the arena, according to the British Transport Police, the force that protects the Manchester Victoria train station next to the arena. The terminal was evacuated.
Early Tuesday morning, Sky News reported that a bomb disposal team had arrived on the scene as part of the investigation and that the security cordon around the arena had been widened.
Gary Walker, who was at the show with his wife and two daughters, said he “heard a massive bang and saw a flash” just as the concert concluded. He turned and realized that his wife had been hurt. Mr. Walker, who is from the northern city of Leeds, said she had a stomach wound and possibly a broken leg. He said he lay down with her and saw “metal nuts on the floor.”
Ms. Walker was taken to a hospital, Mr. Walker said while standing with his daughters at Deansgate, the main shopping street in Manchester.
Another concertgoer, Sasina Akhtar, told The Manchester Evening News that there had been an explosion at the back of the arena after the last song. “We saw young girls with blood on them,” she said. “Everyone was screaming, and people were running.”
Ms. Grande, a singer with a big voice who started her career as a star on a Nickelodeon TV series, is on an international tour supporting her 2016 album, “Dangerous Woman.” Two additional acts, Victoria Monét and Bia, performed as openers on Monday. The tour was scheduled to continue on Thursday at the O2 Arena in London.
“Ariana is O.K.,” Ms. Grande’s publicist, Joseph Carozza, said before the enormity of the episode was widely known. “We are further investigating what happened.”
TMZ, the entertainment news website, reported that Ms. Grande was “in hysterics” over the deadly blast.
Parents separated from their children during the mayhem were told to go to a Holiday Inn, where many children had taken refuge. A number of hotels, including the Holiday Inn and the Travelodge, opened their doors to concertgoers trapped inside the police cordon, providing them with drinks and phone chargers to enable them to contact family members. Residents also offered stranded concertgoers places to stay in their homes.
The confusion and fear in the hours afterward was reflected on social media. One Twitter post asked: “Did anybody see my girlfriend? I lost her in the chaos.”
The BBC interviewed one witness who was waiting outside the arena to pick up his wife and daughter. He recounted that the “whole building shook,” that there was “carnage everywhere,” and that the explosion appeared to come from near the stadium’s ticket area.
Videos posted on Twitter showed concertgoers running and screaming. Hannah Dane, who attended the performance, told The Guardian that she had heard “quite a loud explosion.”
“It shook,” she said. “Then everyone screamed and tried to get out.”
The Manchester Arena, opened in 1995, can hold up to 21,000 spectators; it was not clear how many people were in the crowd for the concert.
Karen Ford, a witness, told the BBC that she had been leaving the concert when the blast occurred. “Everyone was just getting out of their seats and walking toward the stairs when all of a sudden a huge sound, which sounded like an explosion, went off,” she said.
“Everyone tried to push people up the stairs,” Ms. Ford recalled, adding that in the chaos, people tried to push past a woman in a wheelchair as children screamed.
She said there was no smoke, just one very loud bang. “It was very, very loud,” she said, adding that her husband thought he had heard a second explosion. “There were shoes on the floor,” left behind by people who had fled, she recalled.
“Just chaos,” she added. “I was trying to tell people to calm down.” She said the masses of people trying to flee created a perilous situation: “We were being crushed.”
Outside, Ms. Ford said, parents awaited children who had attended the concert, checking their smartphones in a panic. “Everyone was trying to find each other,” she said.
While the country and the world reacted to the news of the explosion and deaths with dismay, anger and grief, the British authorities, who have foiled numerous terrorist plots, were probably not surprised.
The terrorist threat level set by MI5, the domestic intelligence service, has been at “severe,” the second-highest level, for months now, meaning officials considered an attack “highly likely.”
While disenchanted young people can be radicalized through extremist websites, officials are particularly worried about the return of hundreds of battle-trained fighters who left Britain and other European countries to join jihadist groups in Syria and Iraq.
Britain is also in the middle of a general election campaign, with a vote due on June 8, and a terrorist attack, as Manchester appears to be, is bound to become part of the political discourse.