Nearly 80 people were injured, nine seriously, when part of a theatre ceiling collapsed during a packed West End show in London.
Eyewitnesses described "chaos and panic" as 10 sq m of plaster and masonry crashed on to people seated in the stalls 40 minutes into a performance of The Curious Incident of the Dog in the Night-Time.
They said there was a loud creaking before the cave-in, which left the Apollo Theatre engulfed in thick dust and debris. One witness reported hearing an actor yell "watch out" to the audience.
Structural engineers and a district surveyor from Westminster City Council are carrying out a structural assessment of the building. They are due to give an update on their investigation this afternoon.
Some onlookers said water dripped through the ceiling before the collapse, raising speculation that heavy rain may be one line of inquiry. There were storms and lightning strikes in the capital at the time of the incident.
Nimax Theatres , which owns the Apollo, described the collapse as a "shocking and upsetting incident", and said its thoughts were "with the audience and staff".
Police commandeered three London buses to take the injured to hospital, many of whom were described as "walking wounded". A makeshift triage centre was set up at the nearby Gielgud Theatre.
London Ambulance Service said it treated 79 patients, of whom 56 were taken to four hospitals. Of those, 47 had suffered minor injuries and nine more serious injuries.
Chief Superintendent Paul Rickett, of the Metropolitan Police, said there was "no suggestion at this stage that (the collapse) was as a result of a criminal act".
There were around 720 people watching the show when disaster struck at 8.15pm on Thursday night.
Martin Bostock, who was in the lower stalls with his family, received a head injury after he was hit by falling debris.
He told Sky News: "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.
"It was complete chaos in the theatre. We got out with cuts and bruises. I think most people did."
School worker Hannah George, 29, said: "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."
The collapse took place as a reference was made to waves crashing on Brighton rocks, leading some in the audience to believe the noise was part of the show.
Mark Haddon, author of the book which the play is based on, tweeted: "I hope that those who were seriously injured are ok. I'm sorry, too, that so many people went through such a terrifying experience. I'm hugely relieved that no-one has died."
Mark Tait, an actor in the production, tweeted: "Thoughts are with all the audience. Horrific and unbelievable."
The Society of London Theatre - which represents producers and theatre owners - said in a statement: "The exact cause of the incident is still being investigated and the theatre owners, Nimax, are working closely with the relevant authorities to establish exactly what happened.
"Our theatres entertain over 32,000 people in central London every night and all theatres take the safety of their audience, performers and staff very seriously.
The theatre - a Grade II-listed building - was completed in 1901 and seats 775 people over four levels.
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