“We are currently treating this as a terrorist incident until we have further information,” said Ian Hopkins, chief constable of Greater Manchester Police.
Hopkins said the blast happened at the conclusion of a concert by American pop star Ariana Grande, and that local police were coordinating with national counterterrorism and intelligence authorities.
“Broken. From the bottom of my heart, I am so, so sorry,” Grande wrote on Twitter after the blast. “I don’t have words.”
British Prime Minister Theresa May said her thoughts are with the victims of what she called an “appalling terrorist attack.”
The explosion took place in the lobby of the 21,000 seat Manchester Arena, which is the largest indoor venue in Europe.
Police later conducted a precautionary controlled explosion near the arena, but said that object turned out to be an item of clothing and not anything suspicious.
Video from the concert showed thousands of concertgoers scrambling and screaming, trying to escape the building. The incident caused transport chaos, with traffic jams outside the venue and rail services being cancelled.
A number of audience members were seen walking around the building covered in blood.
Many in the audience were young girls who are fans of Grande, a singer and actress who has appeared in TV and film roles. A spokesman for her record label said the singer is “OK.”
While no one reported seeing any smoke, some say the ground near the blast was covered with nuts and bolts.
Abandoned shoes, phones and jackets were scattered throughout the arena.
“It was a huge explosion. You could feel it in your chest. It was chaotic. Everybody was running and screaming just trying to get out,” a concertgoer told Reuters.
Worried parents who brought their children to the show crowded the streets outside the building. A nearby hotel opened its doors to the kids looking for their mothers and fathers.
Cab drivers turned off their meters and offered to drive people from the ill-fated concert to wherever they want to go.
Britain’s terrorism alert level is at “severe,” the second highest on the scale. Terrorism officials are meeting in London.
The U.S. Department of Homeland Security said it was monitoring the situation in Manchester, and that it did not have any information showing a “specific credible threat” to music venues in the U.S.
Grande’s “Dangerous Woman Tour” is to support her third studio album, Dangerous Woman.
The tour began on February 3 in Phoenix, Arizona. From Manchester, the tour is to move through Europe, including Belgium, Poland, Germany, Switzerland and France, through the summer with stops in Brazil, Chile, Argentina, Costa Rica, Mexico and on to Japan, Thailand, the Philippines, New Zealand and Australia.