Apollo Theatre collapse: We thought cracks and bangs were part of the show – but the cast stopped and looked upwards in horror

Independent feature writer Simon Usborne didn't appreciate at first how lucky he was to be seated so near the back of the theatre

Emergency services said nearly 90 people had been injured in a packed London theatre when part of the ceiling collapsed during a performance, bringing the city's West End theatre district to a standstill

It began with a dash to the theatre in the rain. I had been in Chinatown for an early dinner with Jess, my girlfriend. Earlier yesterday she had revealed my first Christmas present - tickets to The Curious Incident of the Dog in the Night-Time.

I had started but not finished the book years ago, and was keen to see the story to its end. I didn't want to be late.

Families, couples and tourists filled the Apollo to capacity - a typical West End audience drawn to a production with excellent reviews. Within minutes, the bustle and anticipation of the week before Christmas dimmed with the house lights, and the titular dog appeared on stage, speared to death with a garden fork.

We were at the end of row Q, two from the back, just by the exit. I didn't appreciate then that these would be the best seats in the house.

We met Christopher, a gifted but challenged teenager who becomes obsessed with the dog's demise. The stage was an open, three-sided box with walls of illuminated graph paper. Lights illustrated the geography of the boy's street, and the production included the innovative use of sound effects.

Soon a policeman entered through the audience, squeezing between rows in front of me to reach the stage. This one was an actor - his character called by the dog's owner. After about 40 minutes, the actress playing Christopher's late mother made the same unconventional entrance. Moments later, a series of cracks and bangs sounded out from above. We thought for a split second they were part of the show, but then the cast stopped and looked upwards in horror.

Another split second and the stage, as well as the entire theatre beyond the shelter of the dress circle above me, disappeared behind a plummeting curtain of dark-grey dust and debris. The thunderous cracks and bangs continued as it fell. They were not sound effects. I'm writing this at midnight and can picture it exactly; the ceiling rained down in violent plumes. It was like watching the back of a waterfall - or an avalanche - from inside a cave in a cliff.

Another split second, and instinct took over. There was no need for an alarm or a call to evacuate. There was fear, both primal and rational - what if the collapse continued. Without taking the time to look at each other, Jess and I, along with everybody who could, shot up and ran for the exits as bangs gave way to screams. Few people grabbed their belongings.

Two more seconds and I reached the pavement on Shaftesbury Avenue, bounding up steps and bursting through a fire exit. We were among the first out, thankfully free of the panicked scrum still trying to leave inside. We ran across the road and only then looked at each other, breathing deeply, hearts racing, swearing.

We were safe but didn't know then what kind of debris that dust contained, or whether people were dead inside. There was genuine fear on that pavement that drama had turned into tragedy.

More people began to emerge, many caked entirely in dust, and with bloodied heads and necks. A mother grabbed her sobbing child by the face and told him he was OK. Within five minutes the first fire engine arrived. Passers-by took in the scene with hands held to their faces.

Shock soon mixed with my instinct as a journalist and I phoned my editors, my voice breaking slightly as I told them what had happened. I told them it could be very serious. Then, as firemen and police began arriving in greater numbers, I walked back across the road and into the theatre's small foyer.

One man lay on his back on the floor, not moving. Another wore an oxygen mask, his face black with dust. A woman, stunned, sat with a bloody bandage tied around her head. She had been sitting in the dress circle above me, she said, and a chunk of plaster had fallen on her head. She held out her hands to show its size - about a foot long.

I asked a senior fire officer whether he knew if there were serious injuries. It was too early to say, he said, but he expected there were. This was not the partial collapse of one part of a box, say, or a single ceiling rose - tonnes of material had come down. He said some people were being treated in their seats.

Outside, one couple stood side by side, staring straight ahead and unable to speak. One woman came out of the neighbouring theatre, where she had been sheltering, and began screaming hysterically. A policeman bandaged a young man's head in the Italian restaurant across the road, where Jess had been given a stiff drink. Young ushers were in tears, calling out for colleagues who had been working beneath the collapse.

A crowd built before the police shut down the street, pushing us back down side streets into Chinatown. By then news crews had mustered, and, thanks to my tweets, my phone went mad with calls from media as far away as New Zealand and Canada until its battery died.

About an hour after the collapse, Jess and I walked fast to Green Park to get the Tube home to Brixton. Reading the news on Jess's phone on the way, it was becoming clear to our great relief that deaths were unlikely and that serious injuries would be mercifully few. The emotional damage to those dust-coated children sobbing on the pavement may take longest to repair.

Still dazed on the train, among Christmas partygoers who had no idea what had happened, I got home to face more calls, from morning TV news producers as well as concerned friends and family. Like Jess and me, they could not believe a festive night at the theatre had ended so abruptly with a scene that had come straight from a disaster movie.