LONDON - The ceiling of a London theater partially collapsed Thursday night, showering a packed audience with heaps of plaster, wood and dust. More than 80 people were injured - at least seven seriously - and several trapped theater-goers had to be rescued, authorities said.

The collapse at the Apollo Theatre took place at 8:15 p.m. during a performance of "The Curious Incident Of The Dog In The Night-Time" at the height of the Christmas holiday season. The ceiling came down, bringing parts of the theater's balconies with it, police said.

"We originally thought it was sound affects of the theater and then we just looked up. It was like slow motion … the whole ceiling came down," said one theatergoer to CBS News. "One of the actors said, 'Watch out, watch out,'" said another theater patron. ""We thought it was part of the play. Then debris, dust and stuff everywhere ... You couldn't see anything, you didn't know what was going on."

More than 700 people were in the theater at the time, according to the London Fire Department.

Emergency workers would not speculate on why part of the ceiling collapsed.

"Complete chaos" erupted as the debris rained down, said Martin Bostock, who came with his family to see the show, which is based on the best-selling novel by Mark Haddon.

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

Libby Grundy, 65, said she heard a bang - and then saw a "huge cloud of dust."

"I thought it was a special effect," she said. "And then people realized it must be some sort of emergency and people started getting up. People didn't panic. (But) people were quite shaky when they got out."

Dust-covered theatergoers, many with bandaged heads, were treated by dozens of emergency workers in the street outside the Apollo and at a nearby theater. London's Ambulance Service said it treated 88 patients - 81 with minor injuries and seven with "more serious" injuries who were taken to nearby hospitals. None of the injuries were believed to be life-threatening, officials said.

A London city bus was commandeered to take some of the wounded to the hospital.

The fire department reported that all those who had been trapped in the Apollo have been rescued.

Shaftesbury Avenue, normally one of London's busiest streets and teeming with pedestrians, was completely shut down by emergency workers.

The Apollo Theatre, named for the Greek and Roman god Apollo, god of music and the arts, was built in 1901 and has 775 seats.

The show, which is aimed at young people as well as adults, is about a boy with Asperger's who sets out to solve a crime.

Prime Minister David Cameron said via Twitter that he was being updated regularly on the crisis. He praised the city's emergency services - who were on the scene within three minutes - for their "fast work" in helping the injured.

A woman stands bandaged and wearing a blanket given by emergency services following an incident at the Apollo Theatre, in London's Shaftesbury Avenue, Thursday evening, Dec. 19, 2013, during a performance at the height of the Christmas season.

A man receives medical attention after part of the ceiling at the Apollo Theatre collapsed in central London on Thursday, Dec. 19, 2013.