Audience members were trapped beneath the rubble after tonnes of plaster from the roof of the 103-year-old Apollo in Shaftesbury Avenue rained down onto the dress circle.
About 720 people were watching The Curious Incident Of The Dog In The Night-time, which was mid-way through a performance.
Khalil Anjarwalla said he, his pregnant wife and her parents managed to escape from the upper circle after "kilos of concrete plummeted from the ceiling".
He said: "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."
Playgoers were escorted out of the building covered in dust, debris and blood.
Some audience members assumed the noise was part of the show, but were alerted by the actors who stopped the play, with one man reporting there was a sudden "crashing noise and part of the roof caved in."
Andrew Howard-Smith, 68, said: "I saw the edge of the balcony come down. 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."
Libby Grundy, 65, added:"There was a bang, and then a huge cloud of dust. At first I thought it was a special effect.
"I heard somebody on the stage say 'Oh, bloody hell' .
"And then people realised it must be some sort of emergency."
London Fire Brigade sent eight fire appliances, sealing off the world-famous theatre street. A spokesman said four people were seriously injured and there were 81 walking wounded.
The Apollo is a Grade Two listed building, and seats 755. The balcony on the third tier is considered the steepest in London.