Eyewitnesses say audience members were standing on temporary boards covering a theatre's orchestra pit when it collapsed, injuring 20 people in Birmingham.
Some fans had dressed in costume for the Sound of Music show
Health and safety investigators are interviewing members of the audience, who were dressed as nuns for a production of Sing-Along-A-Sound of Music.
A spokesman for the Alexandra Theatre spokesman told Reuters news agency the injured audience members had fallen through a cover at the front of the stage.
He said: "They had been invited up on the stage and fell through a cover that was over the orchestra pit."
Eleven people were taken to three city hospitals, many suffering from fractures of the arms and legs.
The 1,347-seat theatre was packed during a warm-up session when the accident happened just before 2000 BST Tuesday.
Members of the audience, who are encouraged to turn up to the show dressed as nuns, members of the Von Trapp family and Julie Andrews, had been invited up to allow a compere to judge a "best-dressed nun contest".
Audience member Julie Wadeson, from Sutton Coldfield, said it was the area which covered the orchestra pit in the Edwardian theatre which collapsed.
She said: "There were so many up there.
"There were about 30 or 40 people on the stage and as they walked forward to the area which covers the orchestra, the stage collapsed.
"They were on the boards that cover the orchestra pit and all of a sudden there was a loud crack and the stage collapsed.
"For a second I think people thought it was part of the show, but then the realisation set in."
The performance normally includes a screening of the 1965 classic film with subtitled song lyrics for the audience to join in and sing along.
Inspector Paul Ford from West Midlands Police said about 20 people fell about 20 feet into the orchestra pit.
He said that when he arrived: "Paramedics were reassuring people and it was very professionally dealt with.
"Fortunately there aren't that many serious injuries, it could have been a lot worse."
Sonia Saxon, an intensive care unit nurse from Birmingham's Heartlands Hospital, who was dressed as a nun, said: "We were rushing down to the front, as the people on stage had called forward all of those dressed as a nun.
"I heard what I thought was a loud bang and just assumed that it was part of
the performance, but could then see that the stage had collapsed.
"People fell straight into the orchestra pit - it must have been a 20-foot drop.
"Everyone was very shocked, but the management acted straight away and called out for any doctors in the audience to rush forward.
Leg and arm fractures
"It seemed like several people had broken bones and all of those who fell,
which must have been over 20 were very shocked."
Steve Evans, a spokesman for West Midlands Ambulance Service, said those injured had suffered a variety of injuries including leg and arm fractures and were taken to Birmingham's Heartlands, Selly Oak and City hospitals.
He said: "Mostly it was minor injuries. There were fractures and some nasty broken leg and arm injuries."
Genital show goes on
Mr Evans said when ambulance crews arrived at the scene, the floor which extended the stage over the orchestra pit had collapsed.
Peter Evans, a manager for Clear Channel, the company which operates the theatre, said people with tickets for Wednesday's show, Puppetry of The Penis, would still be able to see the performance.
A spokeswoman for Birmingham City Council, whose environmental health
department is investigating the accident, said Wednesday's show - billed as the Ancient Australian Art of Genital Origami - would go ahead.
She explained: "We understand that this show only involves two people who will be performing well back from the area under investigation."