Tension and chaos gripped London and the rest of the United Kingdom on Thursday as one of its oldest theatres collapsed during a sold-out event.

BBC reported that at around 8:15 PM, the ceiling of 750-seater Apollo Theater, which was built in 1901, crumbled while the highly anticipated show The Curious Incident of the Dog was being performed. At least 81 people were confirmed injured and the authorities are expecting this number to go up as a score of people remain trapped inside. Seven of these victims were critically wounded and were rushed to the nearest hospital.

At least 25 land and air ambulance rushed to the scene and Shaftesbury Avenue, the road where the Apollo Theatre lies, was immediately shut down to motorists.

Police said the ceiling fell down, dragging down other parts of the theatre which at that time was occupied by more than 700 people. Investigators are yet to make a statement regarding the cause of the Apollo Theater's melt down.

Martin Bostock, who was with his family while watching the show, described the event as 'complete chaos,' adding that no one in the building has an idea why it happened.

"At first, we thought it was part of the show," he told Sky News. "Then I got hit on the head."

Another spectator, Libby Grundy, said she saw huge cloud of dust after hearing a loud bang.

I thought it was a special effect," she said. "And then people realized it must be some sort of emergency and people started getting up. People didn't panic. (But) people were quite shaky when they got out."

On Twitter, Prime Minister David Cameron said he is tracking the progress of the search and rescue operation and hailed London's Ambulance Service - who were on the scene within in only three minutes - for their "fast work" in treating those who were wounded.