London (CNN) -- The hundreds of theatergoers who packed into Central London's historic Apollo Theatre on Thursday night were expecting to watch a mystery.
But about 40 minutes into the play, shortly after 8 p.m., they got a drama instead.
"One of the actors said, 'Watch out!'" said one woman. "We thought it was part of the play."
It wasn't. Instead, it was part of the century-old structure's ornate plaster ceiling, which tumbled five stories onto the theatergoers, injuring scores of them, seven seriously, officials said.
"We heard a creak, somebody screamed, somebody from over there said, 'Look out!' and then the ceiling kind of creased in the middle and then just collapsed," said Hannah, who with her husband had snagged one of the last pairs of tickets to the performance in the 775-seat theater.
Though she said she herself felt "slight panic," she credited those around her for keeping their calm. "It wasn't every man for himself," she said, noting that several of her fellow theatergoers checked on those around them. "Everyone was looking out for each other, and in a couple of minutes, everyone was out."
The London Fire Brigade's Kingsland Station Manager Nick Harding said about 720 people were inside when a section of the ceiling collapsed on the theatergoers, taking parts of the balconies with it.
An ambulance official said a total of 76 people had been injured, though many of them were treated at the scene and released.
"It was like slow motion," said one girl, who initially thought the sound of the falling ceiling was sound effects, but soon learned otherwise. "It just kinda came down."
Harding said what fell was a 10-meter-by-10-meter square section of the ceiling, and that it dragged down balconies with it.
"You initially thought it was part of the play and then you could feel things on you," said an older woman. "The dust that came down -- you couldn't see in front of you."
Metropolitan Police said those who were seriously hurt had been taken to hospitals in central London. None of the injuries appeared life threatening, one official said.
Within an hour, a spokesman for the fire department said everyone, including those who had been hurt, had been evacuated from the building.
Martin Bostock, who was with his wife and two children, said he thought the cave-in was part of the show until something very hard hit him on the head and chaos and panic erupted.
"You couldn't see across the room because of the dust, which we were all breathing in," he told CNN. "It was absolutely horrific and very terrifying. I was with my wife and two kids. Thank God, we got out."
The Apollo is located next to Piccadilly Circus in the Soho district, which is usually packed with tourists, shoppers and diners at theater time.
Within minutes, paramedics arrived carrying stretchers as police cordoned off the area.
A few minutes later, some of the paramedics emerged from the theater, their stretcher full; others helped someone limp out of the building.
Many of the injured were taken initially to the nearby theaters for an initial assessment of their condition by medical workers.
The initial police assessment of the collapse was that it was an accident. "There is no suggestion at this stage that this was as a result of a criminal act, however, at this stage we are keeping an open mind," Metropolitan Police said in a statement.
Outside the Apollo, the street was packed with dozens of police, fire and ambulance vehicles as well as well-dressed theatergoers, some of their faces darkened with dust.
"The Curious Incident of the Dog in the Night-Time," which was adapted to the theater from the 2003 mystery novel by British writer Mark Haddon, is about a 15-year-old boy's investigation into the death of a neighborhood dog.
Ticket prices at the four-level theater, which opened its doors in 1901, included a £1 ($1.64) "theatre restoration levy," the theater's website said. It was not immediately clear whether the ceiling was part of the restoration of the building, which is owned and operated by Nimax Theatres.
In a statement, a spokesman said the company would "continue to cooperate fully with the authorities to establish exactly what happened tonight."
Prime Minister David Cameron, in a tweet, commended the fast work of emergency services.
Now, what started as a mystery and veered into drama has again become a mystery, as engineers try to determine what caused the collapse.
CNN's Nic Robertson in London reported, Tom Watkins in Atlanta reported and wrote, Erin McLaughlin in London and Dana Ford in Atlanta contributed to this report