Apollo Theatre, one of the popular venues in West End theatre district, was packed with about 720 people for a show of "The Curious Incident of the Dog in the Night-Time", when plaster and masonry from a section of the ceiling fell with a crackling sound and left many wounded, including some with head injuries.
Paramedics initially said 88 people were injured but later revised the number to 76, seven of whom were seriously wounded. Another 51 "walking wounded" were taken to hospital with minor injuries.
Some reports suggested that water began dripping through cracks in the ceiling before it came down.
There had been suggestions that the collapse may have been caused by a freak lightning strike, but London Fire Brigade said it had no records on its internal log of lightning.
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London was hit by a freak thunderstorm last evening, which seems to have resulted in the incident at the building but the exact cause of the collapse is yet to be confirmed.
Deputy Assistant Commissioner Graham Ellis of the London Fire Brigade said the plasterwork had fallen onto the upper circle, the dress circle and the stalls.
"The injuries to the casualties that our colleagues have spoken about are consistent with that. We've managed to stabilise the situation inside the theatre. We're working closely with colleagues at Westminster City Council, their building surveyors," he said.
A district surveyor from the local council examined the damaged roof of the Grade II listed building and confirmed it was not at risk of caving in further.
Investigations are still underway into what caused a large section of ornate plasterwork to collapse.
A spokesperson for Nimax Theatres, which owns the Apollo, said their "thoughts are with the audience and staff".
The theatre's owner described the incident as "shocking and upsetting".