A rescue operation is under way after part of a balcony in the Apollo theatre in London's West End is thought to have collapsed during a performance, trapping people inside, according to eyewitnesses.
Police confirmed they were called to the theatre in Shaftesbury Avenue shortly after 8.15pm.
A spokesman said: "There is a report of a collapse in the building itself."
It is believed some people may have been injured in the collapse, which occurred during a performance of The Curious Incident Of The Dog In The Night-time.
Eyewitnesses said they saw people being escorted out of the building, covered in dust and debris.
Halfway through the first half of the performance, part of the balcony started creaking before a section of the theatre collapsed.
Audience members assumed the noise was part of the show.
Rupert Street and Shaftesbury Avenue were filled with evacuated theatre-goers covered in dust, many of whom were bleeding.
People left the building crying, coughing and helping each other away.
Many theatre-goers were crying and trying to make contact with family members as some were still trapped inside the building.
One 29-year-old, who would only give his name as Ben, said: "It was about halfway through the first half of the show and there was a lot of creaking.
"We thought it was part of the scene, it was a seaside scene, but then there was a lot of crashing noise and part of the roof caved in. There was dust everywhere, everybody's covered in dust.
"We got out fairly quickly, I think everyone was quite panicked."
A 38-year-old said: "We were in the stalls. It's a balcony that's come off. Some of the structure's come down."
Police were on the scene within minutes and began cordoning off the theatre.
A Metropolitan Police spokesman said there were "multiple casualties".
London Fire Brigade confirmed that eight fire engines had been sent to the scene .
Martin Bostock, who was in the audience at the Grade II listed theatre with his family, said he suffered a head injury after he was hit by falling debris.
He told Sky News: "I was in the lower stalls with my family in the early stages of the show.
"It was just terrifying and awful.
"I think the front part of the balcony fell down.
"At first we thought it was part of the show.
"Then I got hit on the head."
Mr Bostock confirmed he had a blood injury and needed medical treatment.
He went on: "It was complete chaos in the theatre. Absolutely terrifying and awful.
"We got out with cuts and bruises. I think most people did."
Walking wounded were taken from the scene in ambulances as a team of firefighters rushed through the front st age door in Archer Street.
Some were taken to the nearby White Horse pub, while others were taken from the scene to be reunited with friends and family members.