AT least five people have been seriously injured and dozens more hurt after part of a balcony at a West End theatre collapsed this evening.

Eyewitnesses described the chaos and panic as a theatre began to crumble down around them.

Mmasonry from the Grade II-listed Apollo theatre plummeted into the stalls below, striking members of the packed audience and filling the theatre with clouds of thick dust.

A capacity audience of more than 700 people was inside the theatre, in Shaftesbury Avenue, London, which was 45 minutes into the National Theatre's performance of The Curious Incident Of The Dog In The Night-time when members of the audience started screaming as it appeared parts of the ceiling caved in.

Eyewitnesses spoke of hearing a loud creaking which some initially thought was part of the show.

Theatre-goer Khalil Anjarwalla said he, his heavily pregnant wife and her parents managed to escape from the theatre safely after kilos of concrete plummeted from the ceiling.

Business owner Mr Anjarwalla said: "I was in the upper circle with my family when, about 45 minutes in, people started shouting and screaming.

"We thought it was part of the play. But the ceiling was crumbling.

"Within an instant the whole roof seemed to come down.

"We saw a lot of people completely covered in dust - I could hardly breathe."

A Metropolitan Police spokesman said on Twitter: "Five people seriously injured. They have been taken to central London hospitals.

"Not aware of any fatalities at this early stage.

"We believe there are more than 40 walking wounded being treated at Gielgud Theatre."

Later reports from the London Fire Brigade described 81 people as walking wounded and as many as seven being taken to hospital to be treated for more serious injuries.

Police said a London bus was used to take the casualties to hospital, while members of the public were asked to avoid the area.

Emergency services were called to the scene shortly after 8.15pm, as eyewitnesses reported many theatre-goers crying and trying to make contact with family members as some were initially thought to be trapped inside the building.

Jess Bowie, content editor of The House magazine, tweeted: "Was just seeing The Curious Incident in the West End when the roof of the Apollo Theatre caved in. Absolutely petrifying.

"Don't know if anyone is trapped in there but people outside are covered in dust and some in blood. Utterly horrible."

Andrew Howard-Smith, 68, said: "I saw the edge of the balcony come down, thats what I saw. We were on the balcony below.

"In the production you had to hold on to the rail and lean over to see what was going on, and we were doing the same.

"Everybody must have got hold of the brass rail and just pushed it over, and then the edge came off. That was the only bit that came off, just the edge."

The London Fire Brigade said the theatre was almost full, with about 700 people watching the performance.

A district surveyor from Westminster City Council was on the way to the scene to carry out structural assessment of the building.

It is thought the engineer, who will be accompanied by police officers, will spend most of the night getting access inside the theatre to ensure it is safe.