The accident happened during a performance Friday by Swedish disc jockey Avicii at the Globe Arena.

Police say about 30 people fell when the floor of the structure caved in. Police spokeswoman Inger Qvennerstedt says about 20 concertgoers aged between 15 and 17 were taken to hospital. She says most of the injuries were minor, but one was “very serious.”

Organizers say a second performance by Avicii scheduled for Friday was canceled. The cause of the collapse was not immediately clear.