(RNN) - London theater-goers reported hearing a crackling sound they thought was part of the show before part of the structure of the Apollo Theater collapsed in on them, injuring dozens.
London ambulance reports they treated 76 patients, most considered "walking wounded." That number was downgraded from an initial report of 88 injured. Seven people sustained serious injuries.
So far, no fatalities have been reported.
"It wasn't a full-roof collapse of heavy-duty beams. It seems to be confined to quite thick ornate plaster that you would find in a theater of that age," said Graham Ellis, Deputy Assistant Commissioner of London Fire Service.
Around 720 people were believed to be in the theater at the time of the collapse, including Simon Usborne, a writer with The Independent, a paper in the UK.
"Back home after Apollo theatre collapse. Terrifying. Sheltered in stalls by upper circle. Loud bangs, cracks. Thought was part of show," he tweeted.
Martin Bowstock, who was at the show with his wife and children, told CNN that part of the balcony he was sitting on collapsed on the people below.
"It was full of dust, you couldn't see couldn't breathe, it was absolutely terrifying."
The Curious Incident of the Dog in the Night-Time was playing at the Shaftesbury Avenue venue.
Anthony Davis, a London broadcaster, tweeted that triage was set up in the Gielgud Theatre next door.
One witness told BBC she heard a "strange crackling noise" before "the roof just crumpled." The theatre "suddenly went dark" with "dust clouds everywhere", she said.
"Was at the #Apollo Theatre watching a play when the roof came crashing in! Praying for those still inside #London," eyewitness Khalil Anjarwalla said on Twitter.
According to CNN, eyewitnesses reported that people were being escorted out covered in dust and debris.
Nic Robertson told CNN people left in an orderly manner after the collapse.
The theatre, which was built in 1901, sits more than 775 people and has four levels.
The Apollo was almost at capacity at the time of the collapse.
The Curious Incident Of The Dog In The Night-Time has been running in London since August 2012.