Seventy-six people were injured, seven seriously, after part of London's Apollo Theatre collapsed as an audience of around 700 watched a performance of The Curious Incident of the Dog in the Night-time.
Audience members with blood streaming from their heads were seen pouring out into the lobby of the West End theatre shortly after 8pm.
Police said a London bus was commandeered to take some casualties to hospitals in the area. Others were treated at the nearby Gielgud Theatre. There were no fatalities.
A district surveyor from Westminster City Council carried out a structural assessment of the building during the night to ensure it was safe, and is expected to report on preliminary findings at 9am, the BBC said.
Kingsland Fire Brigade station manager Nick Harding said: “A section of the theatre’s ceiling collapsed onto the audience who were watching the show. The ceiling took parts of the balconies down with it.
“Firefighters worked really hard in very difficult conditions and I’d like to pay tribute to them. They rescued people from the theatre, made the area safe and then helped ambulance crews with the injured.”
“In my time as a fire officer I’ve never seen an incident like this. I imagine lots of people were out enjoying the show in the run-up to Christmas. My thoughts go out to all those affected.”
He added that specialist urban search and rescue crews made sure no one else was trapped.
The theatre, which opened in 1901 and is a Grade II listed building, seats 755 on four levels and the balcony on the third tier is considered the steepest in London. The incident occurred an hour after the city was hit by a large thunderstorm, although it is not clear if this contributed to the collapse.
Simon Usborne, a feature writer at The Independent, was on the ground floor of the theatre's sheltered upper circle when the collapse happened.
“There was a series of loud cracks and bangs,” he said, “but the show is full of sound effects and for a split second I think a lot of people thought this was something to do with the show.
"Then my vision was instantly blocked by a huge curtain, a huge cloud of falling dust and debris. The sound was still going on.
“We grabbed what we could and ran for the door. There was panic, there was screaming.”
Outside he said was a scene of human misery, with children and adults crying and screaming and some bleeding from head injuries.
Others were “in a state of shock” and most people were covered in the black and grey dust.
“I went back into the foyer and saw a dozen or more people either sitting or lying there waiting to be treated,” Mr Usborne said.
“There was a woman who said a foot-long chunk of plasterwork fell on her head. She had been bandaged and was bleeding.
"There was one couple I met who were totally glazed over and barely responded to my question.“
He said everyone seemed to be "pretty shaken up".
Andrew Howard-Smith, 68, said he saw the edge of a balcony come down.
"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," he said.
"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."
Prime Minister David Cameron tweeted that he had been “updated regularly on the Apollo incident”.
“I'm grateful for the fast work of the emergency services in helping the injured,” he said.
School worker Hannah George, 29, said people were able to exit the building quickly and orderly: “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.”
London Ambulance Service incident commander Maria Smith said: “When I arrived it was dark and extremely dusty - people were lying on the floor of the theatre.
“We very quickly set up a casualty clearing area in the foyer of the theatre - the walking wounded were assessed and treated there.”
Simon Stephens, the playwright working on the production, thanked people on Twitter for messages of support “on this sad and strange night”.
Matt Tait, an actor in the show, tweeted that the cast, crew and stage management were “all safe”.
“Thoughts are with all the audience. Horrific and unbelievable,” he said.
Audience member Dee Kearney said she was about three rows back 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 said.
“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.”
Jess Bowie, content editor of The House magazine, who was also in the audience, said it was "petrifying".
"Don't know if anyone is trapped in there but people outside are covered in dust and some in blood. Utterly horrible," she wrote on Twitter.
The actor Matt Lucas tweeted shortly after the collapse: "Just seen the news about the Apollo Theatre. Everyone there in my thoughts."
London Fire Brigade said it sent eight fire engines and three fire and rescue units to the scene with a total of about 60 firefighters.