LONDON — An explosion struck an Ariana Grande concert attended by thousands of young music fans in northern England late Monday, killing at least 19 people and injuring dozens in what police were treating as a terrorist attack.

Greater Manchester Police said 19 people were confirmed dead after the explosion at Manchester Arena. Northwest Ambulance Service said 59 injured people had been taken to hospitals, and a number of “walking wounded” were treated at the scene.

Police cars, bomb-disposal units and 60 ambulances raced to the scene as the scale of the carnage became clear.

“We are currently treating this as a terrorist incident until we know otherwise,” said Ian Hopkins, chief constable of Greater Manchester Police.

There was panic after the explosion, which struck around 10:30 p.m. (2130GMT) as Grande was ending the concert, part of her Dangerous Woman Tour.

Grande, who was not injured, tweeted hours later: “Broken. From the bottom of my heart, I am so so sorry. I don’t have words.”

Manchester Arena said on its website that the blast struck outside the venue as concertgoers were leaving.

Jenny Brewster said she was leaving the concert with her 11-year-old daughter when the blast hit.

“As I turned around, boom, one loud noise,” she told Sky News. “A gentleman said ‘run!’ so we ran.”

Outside, she said, “you could smell the burning.”

Britain’s terrorist threat level stands at “severe,” the second-highest rung on a five-point scale, meaning an attack is highly likely.

“A huge bomb-like bang went off that hugely panicked everyone and we were all trying to flee the arena,” concertgoer Majid Khan, 22, told Britain’s Press Association. “It was one bang and essentially everyone from the other side of the arena where the bang was heard from suddenly came running towards us as they were trying to exit.”

Added Oliver Jones, 17: “The bang echoed around the foyer of the arena and people started to run.”

Video from inside the arena showed concertgoers screaming as they made their way out amid a sea of pink balloons.

British Prime Minister Theresa May said the government was working to establish “the full details of what is being treated by the police as an appalling terrorist attack.”

May is due to chair a meeting of the government’s COBRA emergency committee later Tuesday. She and other candidates suspended campaigning for Britain’s June 8 election after the blast.

If the incident is confirmed as a terrorist attack it would be the deadliest in Britain since four suicide bombers killed 52 London commuters on three subway trains and a bus in July 2005.

Police advised the public to avoid the area around the Manchester Arena, and the train station near the arena, Victoria Station, was evacuated and all trains canceled.

The Dangerous Woman tour is the third concert tour by 23-year-old Grande and supports her third studio album, “Dangerous Woman.”

Grande’s role as Cat Valentine on Nickelodeon’s high school sitcom “Victorious” propelled her to teen idol status, starting in 2010.

The tour began in Phoenix, Arizona, in February. After Manchester, Grande was to perform at venues in Europe, including Belgium, Poland, Germany, Switzerland and France, with concerts in Latin America and Asia to follow.

Pop concerts and nightclubs have been a terrorism target before. Almost 90 people were killed by gunmen at the Bataclan concert hall in Paris during a performance by Eagles of Death Metal in November 2015.

In Turkey, 39 people died when a gunman attacked New Year’s revelers at the Reina nightclub in Istanbul.

Manchester, 160 miles (260 kilometers) miles northwest of London, was hit by a huge Irish Republican Army bomb in 1996 that leveled a swath of the city center. More than 200 people were injured, though no one was killed.

AP writer Leanne Italie in New York and AP Music Writer Mesfin Fekadu in Jersey City, New Jersey, contributed to this report.