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 station 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 on the floor beside her and saw “metal nuts on the floor.”

Ms. Walker was taken to a hospital, Mr. Walker said while standing with his daughters in 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.

Ms. Grande was not hurt. TMZ, the entertainment news website, reported that she was “in hysterics” over the deadly blast.

Her manager, Scooter Braun, said on Twitter, “We mourn the lives of children and loved ones taken by this cowardly act.”