Michael Winter, USA TODAY

LONDON -- Part of the roof of a West End theater collapsed during a packed performance Thursday night, resulting in "multiple casualties" but no fatalities, the Metropolitan Police say.

The collapse occurred at the Apollo Theater about 8:15 p.m. local time (3:15 p.m. ET) during a nearly sold-out performance of The Curious Incident Of The Dog In The Nighttime. The play began at 7:30 p.m.

The London Fire Brigade said that 20 to 40 people were injured, and that an estimated 700 people were in the theater at the time. Rescuers freed several who were trapped under debris.
Police said that five people were transported to a hospital with serious injuries, and that medics were treating about 40 "walking wounded" at the nearby Gielgud Theater.

One witness told the BBC that the "entire dome roof" collapsed on patrons seated in the stalls. Another said she heard "a strange crackling" noise before "the roof just crumpled." Initial reports indicated the balcony had collapsed.

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. It's not known if it was involved in the accident.

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."

Another theater-goer told the BBC that "there was a lot of creaking" during a "seaside scene, and he thought it 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 the award-winning novel by Mark Haddon. The London Times hailed it as a "phenomenal combination of storytelling and spectacle."

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.