Emergency services said 65 people had been injured in a packed London theatre on Thursday when part of the ceiling collapsed during a performance, showering the audience with masonry and debris.

The incident occurred at about 8:30 p.m. at the Apollo Theatre, where many families were among the audience for "The Curious Incident of the Dog in the Night-Time".

The London Ambulance Service said 65 people had been treated, including five taken to hospital with more serious injuries. The London Fire Brigade said all those who were trapped had now been freed.

"We are not aware of any fatalities at this early stage," a police spokesman added.

Witnesses said the ceiling in the four-storey, 775-seat auditorium had simply collapsed during the performance, sending those inside into a panic when they realised it was not part of the play.

"We saw the ceiling give way and it just dropped down onto the stalls. There was dust everywhere and people were screaming," Steve George, 29, who was sitting in seats at the top of the theatre, told Reuters.

They were quickly ushered out of the theatre, he said.

"I wanted to stay to help people ... but I was more in the way, so we left. I have no idea how many people would have been injured," added George, a cinema manager who had taken his wife Hannah to the show for a birthday treat.

A Reuters reporter at the scene said emergency vehicles had blocked off Shaftesbury Avenue in the heart of London's theatreland.

"People were running in here with dust all over themselves," said Thomas Asihen, manager of the McDonald's restaurant located on the same block. "A woman told me she saw the ceiling falling down."

He said he had seen people being brought out by paramedics, with one person on a stretcher. Those injured inside the Apollo, which first opened its doors in February 1901, were taken to another theatre nearby.

Police said the cause of the collapse was unclear, but there was no suggestion that it was the result of any deliberate act or attack.