Theatre-goers have described how actors shouted "watch out!" to the audience as debris began to fall from the ceiling.
An investigation is underway into what caused the collapse at the Apollo in London's West End.
Witnesses described how people did not panic as they filed out of the building.
Andrew Howard-Smith, 68, said: "I saw the edge of the balcony come down, that's what I saw. We were on the balcony below.
"In the production you had to hold on to the rail and lean over to see what was going on, and we were doing the same.
"Everybody must have got hold of the brass rail and just pushed it over, and then the edge came off. That was the only bit that came off, just the edge. It wasn't the whole of the balcony, just the front 2ft."
School worker Hannah George, 29, said: "It was about 40 minutes into the show, and the ceiling collapsed.
"We were in the balcony, about five rows from the front, and we saw a few people in the front of the balcony row get up
and start moving towards the right. Very quickly, the second and third rows started moving all together. We wondered if this part of the show.
"Then I heard someone scream and you heard a shriek - then a chunk of the ceiling collapsed.
"It actually missed the balcony and must have hit people down below in the stalls - you couldn't see anything down there.
"Very quickly ushers held the doors open. It wasn't every man for himself, it was very ordered. There were people in front going, 'You OK?' and trying to get people out.
"There were people coming out who were more seriously injured. There were loads of people coming out shaking, and a fellow next to me had quite a badly bleeding arm and a ripped shirt."
She described people seriously injured, including a man with blood all down his arm and a woman lying on the pavement outside "absolutely covered in glass".
Cinema manager Steve George, 29, said he was level with the section of ceiling which is believed to have given way.
He said: "The room filled with dust straight away.
"I couldn't see the people down below, and the initial thought was that it's given way, it's missed where we're sitting.
But then you realise there's the stalls down there and it's fallen on those people."
He said emergency lights soon came on as people made for the exits.
"People were just walking quickly rather than pushing or shoving, it was fairly orderly," he said.
"We saw the people coming out, all covered in dust and blood.
"Where we were, right at the back of the balcony, you can't even see the stage, so we couldn't see anything in the stalls
"There was a lot of screaming, but people in our area had already sort of seen something and had started moving."
Martin Bostock, who was in the audience with his family, said he received a head injury after he was hit by falling debris.
He told Sky News: "I was in the lower stalls with my family in the early stages of the show.
"It was just terrifying and awful. I think the front part of the balcony fell down.
"At first we thought it was part of the show. Then I got hit on the head."
Mr Bostock added: "It was complete chaos in the theatre. We got out with cuts and bruises. I think most people did."
Sean Walsh, 41, from north London, said: "We were right up in the gods and a couple in the group below just said, 'Go!'
"We thought they were just leaving because maybe they were bored, and my girlfriend thought maybe they had seen a mouse. But then the whole of the ceiling just came down."
Libby Grundy, 65, said: "There was a bang, and then a huge cloud of dust. At first I thought it was a special effect.
"I heard somebody on the stage say, 'Oh bloody hell', because they must have seen it.
"And then people realised it must be some sort of emergency and people started getting up. People didn't panic. People were quite shaky when they got out.
"There wasn't any screaming. People were scared, but they weren't screaming.
"I feel quite shaky now."
Dee Kearney said: "We were three or four seats from the stage when an actor turned around and said 'Watch out!' We initially thought it was part of the play.
"Then what we felt was debris falling on us, a loud bang, and then all of a sudden there was a coat of dust that came on us.
She added: "It was a thick cloud of dust, so you couldn't actually tell what was happening. You knew it was coming from above but you didn't know what was actually happening."