It wasn't immediately clear if the roof, ceiling or balcony collapsed at the Apollo Theatre during a performance of "The Curious Incident Of The Dog In The Nighttime."
Police said officers were called at around 2015 GMT (3:15 p.m. EST) to reports of a ceiling collapse at the theater on Shaftsbury Avenue in central London. It said police "are aware of a number of casualties," but had no further details.
Witnesses told British media that the theater in London's famous West End was packed during the holiday season to see the show, which is based on the best-selling novel by Mark Haddon.
"We thought it was part of the show, until something hit me on the head very hard," one man told the BBC, speaking from the foyer of the theater while he was being evaluated by emergency workers. "I thought we were all going to be in really, really serious trouble."
"Within an instant, the entire roof caved in," another witness told the BBC.London's fire department said eight engines are on the scene in the SoHo neighborhood, and the city's ambulance service said it had sent "a number" of crews to the theater.
Martin Bostock was in the audience with his family, and said "complete chaos" erupted in the theater.
"At first we thought it was part of the show," he told Sky News. "Then I got hit on the head."
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.
London was hit by a freak 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.
On its website, the theatre warned that its balcony was one of the steepest in London "so avoid if you have trouble with heights."