More than 80 people have been injured, seven seriously, after a theatre ceiling collapsed on top of them.
Eyewitnesses described "chaos and panic" as parts of the Apollo theatre in London's West End "began to crumble down" around them.
Many are described as "walking wounded" after masonry and plaster from the Grade II listed theatre plummeted into the stalls below, dragging a section of the balcony with it, striking members of the packed audience and filling the theatre with clouds of thick dust.
More than 700 people were inside the Shaftesbury Avenue venue - which was 45 minutes into the National Theatre's performance of The Curious Incident Of The Dog In The Night-time - when members of the audience started screaming as it appeared parts of the ceiling caved in.
Eyewitnesses spoke of hearing a loud "creaking" which some initially thought was part of the show.
Theatre-goer Khalil Anjarwalla said he, his heavily pregnant wife and her parents managed to escape from the theatre safely after "kilos of concrete plummeted from the ceiling".
Business owner Mr Anjarwalla, 29, said: "I was in the upper circle with my family when, about 45 minutes in, people started shouting and screaming.
" Within an instant the whole roof seemed to come down.
"We saw a lot of people completely covered in dust - I could hardly breathe.
"We had to get out, calmly. I remember thinking the cloud, the dust."
Mr Anjarwalla, who was visiting his in-laws with his English wife, Aliya, said: "The actors just seemed to run from the stage. They had obviously seen what had happened.
"We initially thought it was part of the show. Thankfully we are all OK. My wife is seven months' pregnant but she is OK. We feel very blessed."
He said some people seemed to be "cut quite badly".
London Ambulance said on Twitter it had treated 88 patients - 81 had suffered minor injuries, while there were seven with more serious injuries who were taken to hospital.
Kingsland station manager Nick Harding, from London Fire Service, said: "A section of the theatre's ceiling collapsed on to 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."
Emergency crews said there was no evidence of anybody being stuck inside the building.
Metropolitan Police chief inspector Stephen Manger said police arrived on scene at about 8.15pm.
He said: " People are shocked and shaken but they're being very well looked after by us."
A London bus was used to take the casualties to hospital, while a makeshift triage was set up at the Gielgud Theatre.
A district surveyor from Westminster City Council is expected to carry out a structural assessment of the building during the night
It is thought the engineer, who will be accompanied by police officers, will spend most of the night getting access inside the theatre to ensure it is safe.
Council leader Philippa Roe said: "We are obviously shocked to hear the news and our immediate concerns are with the people that have been injured. Our officers are on site and they will help emergency services wherever they can."
Sean Walsh, who was visiting the show with his girlfriend, said they were sitting in the balcony when they first spotted a group of people below them shouting to leave the theatre immediately.
Mr 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."
He said people in the balcony filed out of the theatre calmly, but added: "It was difficult - you could hardly see the seat in front of you, due to the dust."
One 29-year-old, who would only give his name as Ben, said: "We got out fairly quickly, I think everyone was quite panicked."
Prime Minister David Cameron paid tribute to rescue crews, writing on Twitter: " I've been updated regularly on the Apollo incident. I'm grateful for the fast work of the emergency services in helping the injured."