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LONDON -- Part of the ceiling of a West End theater collapsed during a packed performance Thursday night, injuring nearly 90 people, authorities said.

The collapse, which also damaged balconies, occurred at the Apollo Theater about 8:15 p.m. local time (3:15 p.m. ET), about 45 minutes into a nearly sold-out performance of The Curious Incident Of The Dog In The Nighttime. No cast members were hurt.

Seven patrons were hospitalized with serious injuries, and medics were treating 81 "walking wounded," the London Ambulance Services said. Earlier, the London Fire Brigade said 20 to 40 of the estimated 700 people inside were injured, and police said five were seriously hurt.

Rescuers freed several who were trapped under wood, plaster and other debris.

"In my time as a fire officer, I've never seen an incident like this," Fire Brigade Officer Nick Harding, who manages the Kingsland Station, said in a statement.

ON THE SCENE: Share photos/video of Apollo Theater collapse

The theater, on Shaftesbury Avenue in SoHo, was built in 1901. It has 775 seats on four levels. The balcony of the third tier is considered the steepest in London, the Guardian notes.

Hannah George, 29, an art teacher from East London, was in the fifth row of the balcony with her husband, when she heard a slowly escalating creaking sound. She said a few people in the front row began getting up, and she initially thought they were heading to the bar. But she quickly realized something was wrong and fled.

"The ceiling just crashed down," she said. "If we had been there another five minutes I don't know what would have happened to us."

"It was supposed to be my birthday treat," George added.

Martin Bostock was in the audience with his family, and said "complete chaos" erupted.

"At first we thought it was part of the show," he told Sky News. "Then I got hit on the head."

A woman stands bandaged and wearing a blanket given by emergency services following an incident at the Apollo Theatre, in London during a performance at the height of the Christmas season. It wasn't immediately clear if the roof, ceiling or balcony had collapsed during a performance. Police said they "are aware of a number of casualties," but had no further details. A woman stands bandaged and wearing a blanket given by emergency services following an incident at the Apollo Theatre, in London during a performance at the height of the Christmas season. It wasn't immediately clear if the roof, ceiling or balcony had collapsed during a performance. Police said they "are aware of a number of casualties," but had no further details. Joel Ryan, AP Fullscreen

A woman stands bandaged and wearing a blanket given by emergency services following an incident at the Apollo Theatre, in London during a performance at the height of the Christmas season. It wasn't immediately clear if the roof, ceiling or balcony had collapsed during a performance. Police said they "are aware of a number of casualties," but had no further details. An injured person is lead away from the Apollo Theatre after part of the ceiling collapsed during a packed performance of 'The Curious Incident of the Dog in the Night-Time.' An injured woman is taken towards a waiting ambulance on a stretcher following a ceiling collapse at a theatre in London. Rescue services arrive at the scene of an apparent balcony collapse at the Apollo Theatre in London. An injured woman cries inside an ambulance after fleeing the ceiling collapse. People sit aboard a London bus used for casualties. Police and emergency services personnel assist in operations behind a cordon following a ceiling collapse at a theatre in Central London. Emergency services attend the scene at the Apollo Theatre in London. The ceiling of a top London theatre collapsed on the audience during a performance. Police and emergency services personnel assist in operations as people rescued stand wrapped in emergency blankets behind a cordon following a ceiling collapse at a theatre in Central London. Police and emergency services respond to a ceiling collapse at a theatre in Central London. The incident caused what police said was an unknown number of casualties and leaving terrified theatregoers covered in blood and dust. Some of the Injured are placed into a London bus while authorities deal with the more seriously injured. Injured people, wrapped in emergency blankets, sit in a London bus being used for casualties following a ceiling collapse at a theatre in Central London.

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Another theater-goer told the BBC that he thought the creaking during a "seaside scene was part of the performance. "But then there was a lot of crashing noise and part of the roof caved in," he said.

"There was dust everywhere, everybody's covered in dust. We got out fairly quickly, I think everyone was quite panicked."

Simon Usborne, who writes for The Independent newspaper, said a cloud of dust obscured the stage after some masonry appeared to fall away.

"There was panic, there was screaming," he said.

There were no obvious signs of damage from outside the theater, he added.

The Curious Incident Of The Dog In The Nighttime, staged by the National Theater, is adapted from Mark Haddon's award-winning mystery novel about a 15-year-old "outsider" who calls himself "a mathematician with some behavioral difficulties." The London Times hailed the stage production as a "phenomenal combination of storytelling and spectacle."

The New York Times said the 2½-hour play "may be the most entertaining family drama since the stage version of 'War Horse'," and Thursday night's holiday house included many families.

The Apollo is owned by Nica Burns of Nimax Theatres. Burns is one of Britain's leading play producers. Including the Apollo, Burns owns with her business partner Max Weitzenhoffer six London playhouses: Palace, Lyric, Garrick, Duchess, Vaudeville.

Shaftesbury Avenue sits in the heart of London's playhouse district, and the Apollo is situated equidistant from the Leicester Square and Piccadilly Circus underground stations.

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