Hunks of plaster and dust rained down on a packed audience when the ceiling of a London theater partially collapsed on Thursday night. More than 75 people were injured, seven seriously, authorities said.

The collapse at the Apollo Theatre took place around 8:15 pm during a performance of The Curious Incident of the Dog in the Night-Time at the height of the Christmas holiday season.

Plaster and masonry from a section of the ceiling tumbled down, bringing parts of the theater's balconies with it onto the audience, police said. More than 700 people were in the theater at the time, the London Fire Brigade said.

Officials said most of the injured were "walking wounded" with upper-body injuries and that all were conscious and breathing.

Police and fire officials said it was too soon to say what had caused the collapse but a full investigation is being carried out.

Dee Stephenson said she was seated near the stage and heard the main actor shout, "Watch out!"

"Then you could feel the debris literally coming down on you, and then I got hit on the back by a large piece," she told The Associated Press. "It was a complete dust curtain. You couldn't see."

Scott Daniels, a US tourist from the Dallas area, said he managed to buy a ticket to the acclaimed production just before showtime.

About 40 minutes into the show, he said, he started hearing noises and screaming.

"I thought, maybe this is part of the play," he said. "All of a sudden, plaster starts raining down, huge hunks of plaster The lights went out, and everything filled with dust, and everybody was coughing and choking."

He said he made it out with "a couple scrapes", though he saw others with more serious lacerations.

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.

City buses were commandeered to usher some of the wounded to hospitals.

Injuries ranged from head wounds to cuts and scrapes to breathing problems.

Initially, London Ambulance Service said more than 80 people had been injured. But it later adjusted that number to say it had treated 76 patients, 58 of whom were taken to hospitals.

Of those, 51 suffered minor injuries, and seven had suffered "more serious injuries". There were no fatalities and none of the injuries was believed to be life-threatening, officials added.

Chief Superintendent Paul Rickett said that "so far, we know that a number of items of masonry have fallen down from the ceiling.

"There is no suggestion at this stage that this was the result of a criminal act; however, at this stage, we are keeping an open mind," he added.

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

The Apollo was built in 1901 and has 775 seats.

British Prime Minister David Cameron praised the city's emergency services - which were on the scene within minutes - for their "fast work" in helping the injured.

The Associated Press