MANCHESTER, England — An explosion that may have been a suicide bombing killed at least 19 people on Monday night and wounded dozens 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 it was possibly a suicide bomber who had detonated a nail-filled explosive device.
The scene immediately evoked the terrorist attacks in Paris in November 2015, which included a deadly assault inside the Bataclan concert arena where the Eagles of Death Metal had been playing. But unlike the Bataclan victims, 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 at the Manchester Arena 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 Victoria Station, the train terminus next to the arena. The terminal was evacuated.
Early on 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, and Mr. Walker was 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, 23, 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, said Ms. Grande was reported to be “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. Local residents 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.”
The Greater Manchester Police said in a statement, “There are a number of confirmed fatalities and others injured.”
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,” she 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 said. “I was trying to tell people to calm down.” She added that the crush 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.