The incident happened yesterday at the Apollo Theatre during the West End show "The Curious Incident of the Dog in the Night-time" and comes the week before Christmas when London's theatres are traditionally busy.
Witnesses heard creaking noises in the 112-year-old theatre before the collapse but thought it was part of the show, before a section of the ceiling collapsed and brought down parts of the balconies.
Rescuers commandeered a red London double decker bus to transport some of the wounded to hospital, according to AFP journalists at the scene.
"We have treated 81 walking wounded and seven more seriously injured patients at the Apollo Theatre incident," London Ambulance Service said in a statement.
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There were around 720 people in the audience at the time, the London Fire Brigade said.
"A section of the theatre's ceiling collapsed onto the audience who were watching the show. The ceiling took parts of the balconies down with it," senior firefighter Nick Harding said.
"In my time as a fire officer I've never seen an incident like this. I imagine lots of people were out enjoying the show in the run-up to Christmas."
AFPTV reporters saw several people being stretchered away from the scene.
Others including some with bandages on their heads were being treated in the lobby of the nearby Queen's Theatre, where the musical Les Miserables was showing, which had been turned into a triage centre.
Desmond Thomas, 18, part of a school party watching the show, said they heard noises before the accident.
"Maybe 10 minutes into the performance we heard a tap-tap noise, we thought it was rain," he told AFP.
"There was a crack and then it suddenly seemed to get bigger and suddenly it collapsed. The next thing we knew the whole theatre filled with dust and smoke."