The roof or ceiling of a London theater partially collapsed Thursday night, showering a packed audience of about 700 with heaps of plaster, wood and dust, authorities and witnesses said.

More than 80 people were injured, including at least seven seriously, and several trapped theater-goers had to be rescued.

The collapse happened at the Apollo Theatre on Shaftesbury Avenue at 8:15 p.m. during a performance of “The Curious Incident of the Dog in the Nighttime” at the height of the Christmas holiday season.

“Complete chaos” erupted in the theater as the debris fell, 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.”

Witnesses said audience members were screaming “Get out! Get out!” as they fled the theater and were shaking with fear when they reached the street.

“Within an instant, the entire roof caved in,” another man told the BBC.

A passing double-decker bus was commandeered to transport some of the injured.

Police said that they weren’t aware of any fatalities “at this early stage” and that those who were seriously injured had been taken to hospitals.

Dozens of people were treated at a nearby theater, police said, while the fire department reported that all those who had been trapped in the Apollo have been rescued.

More than two dozen ambulances and eight firetrucks were sent to the scene. Shaftesbury Avenue, normally one of London’s busiest streets and teeming with pedestrians, was shut down.

London was hit by a strong thunderstorm about 7 p.m. that dumped heavy rain on the city, but it wasn’t immediately clear if that was related to the collapse.

The Apollo Theatre was built in 1901 and has 775 seats. The London fire department said it thought about 700 people were in the building at the time.

On its Web site, the theater warns that its balcony is one of the steepest in London, “so avoid if you have trouble with heights.”