London authorities say between 20 and 40 people have been injured at a theater which partially collapsed during a packed performance of a show during the height of the Christmas season.

The London fire department says all casualties who were trapped in the Apollo Theatre have been freed.

It wasn't immediately clear if the roof, ceiling or balcony collapsed at the theater during a performance of "The Curious Incident Of The Dog In The Nighttime" on Thursday evening.

The fire department believes around 700 people were in the theater at the time.

The department added that special search and rescue teams remain on the scene, along with ambulances and police.