Eyewitnesses have described the "chaos and panic" as a theatre "began to crumble down" around them, leaving scores of people injured.
At least five people have been seriously hurt and more than 40 people are described as "walking wounded" after masonry from the Grade II-listed Apollo theatre in London's West End plummeted into the stalls below, striking members of the packed audience and filling the theatre with clouds of thick dust.
A capacity audience of more than 700 people was inside the theatre, in Shaftesbury Avenue, 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 said: "I was in the upper circle with my family when, about 45 minutes in, people started shouting and screaming.
"We thought it was part of the play. But the ceiling was crumbling.
"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 - it reminded me of those scenes from 9/11 in the aftermath of the building collapsing."
The 29-year-old, who is from Nairobi in Kenya, said: "I was very close to the Westgate Mall which was the scene of the recent terror attack, so my first thought was instantly to check whether we should be ducking for safety.
"We then realised it was a structural issue and tried to get out."
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".
A Metropolitan Police spokesman said on Twitter: " Five people seriously injured. They have been taken to central London hospitals.
"Not aware of any fatalities at this early stage.
"We believe there are more than 40 walking wounded being treated at Gielgud Theatre."
Police said a London bus was used to take the casualties to hospital, while members of the public were asked to avoid the area.
Emergency services were called to the scene shortly after 8.15pm, as eyewitnesses reported m any theatre-goers crying and trying to make contact with family members as some were initially thought to be trapped inside the building.
Jess Bowie, content editor of The House magazine, tweeted: "Was just seeing 'The Curious Incident' in the West End when the roof of the Apollo Theatre caved in. Absolutely petrifying.
"Don't know if anyone is trapped in there but people outside are covered in dust and some in blood. Utterly horrible."
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."
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."
One 29-year-old, who would only give his name as Ben, said: "It was about halfway through the first half of the show and there was a lot of creaking.
"We thought it was part of the scene, it was a seaside scene, but then there was a lot of crashing noise and part of the roof caved in. There was dust everywhere, everybody's covered in dust.
"We got out fairly quickly, I think everyone was quite panicked."
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."
The London Fire Brigade said the theatre was almost full, with " around 700 people" watching the performance.
A spokesman added: "It's thought between 20 and 40 people were injured."
A district surveyor from Westminster City Council was on the way to the scene to carry out structural assessment of the building.
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."
London Fire Brigade described 81 people as "walking wounded" with seven taken to hospital with "more serious injuries".