A ceiling has collapsed at London's Apollo Theatre, wounding dozens of people and prompting a swift search-and-rescue operation.

The London fire department said about 700 people were inside the theatre in the British capital when the incident occurred on Thursday evening.

"We saw the ceiling give way and it just dropped down onto the stalls. There was dust everywhere and people were screaming," said Steve George, who was in the audience.

It was hard to see anything through the dust. Martin Bostock, audience member

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

In a message posted to Twitter soon after the incident, the fire department confirmed that all trapped audience members had been freed.

More than 75 people were injured, emergency services said, including seven who were taken to hospital with more serious injuries.

It was not immediately clear why the ceiling caved in.

Mass panic

The incident, which occurred during a packed performance of The Curious Incident of the Dog in the Night-time, showered audience members with masonry and debris.

Emergency vehicles rushed to the scene, blocking off Shaftesbury Avenue, where the theatre is located.

Witnesses said the ceiling in the four-storey, 775-seat auditorium simply collapsed during the performance, sparking mass panic when theatregoers realised it was not part of the play.

"I heard someone scream... Ushers helped people get out," witness Hannah George told Al Jazeera.

Thomas Asihen, who manages a fast-food restaurant located on the same block, said people ran into his business covered in dust, and he watched as others were rescued from the theatre by paramedics.

The Apollo Theatre is more than a century old, having first opened its doors in February 1901.