There was little warning of the chaos that was to come as passengers on Singapore Airlines flight SQ321 relaxed with just three hours to Singapore after a long-haul flight from London.

But as the Boeing 777-300R soared above Myanmar it suddenly hit extreme turbulence, wildly throwing passengers, flight attendants and meals around the cabin.

“Suddenly the aircraft starts tilting up and there was shaking,” Malaysian student Dzafran Azmir said.

The 28-year-old braced himself and checked he had his seatbelt on. He did. Many of the other passengers did not, he said.

“There was a very dramatic drop so everyone seated and not wearing seatbelt was launched immediately into the ceiling, some people hit their heads on the baggage cabins overhead and dented it, they hit the places where lights and masks are and broke straight through it.”

One passenger died of a suspected heart attack and dozens were injured after the flight encountered what the airline described as sudden extreme turbulence around 10 hours into the journey.

“I remember the objects flying, the tightness of my waist from the seatbelt, which was obviously holding me in place,” said Andrew Davies who had just put his belt on after the seatbelt sign illuminated during what had been a “perfectly normal” flight.

There was screaming and a lady with a bleeding head wound, he said.

The flight experienced “a rapid change in vertical rate, consistent with a sudden turbulence event” at 0749 GMT, flight data provider FlightRadar 24 said.

The incident lasted a few seconds, both passengers told Reuters.

“People dropped to the ground, my phone flew out of my hand and went a couple aisles to the side, people’s shoes flung about,” Azmir said.

Oxygen masks hung down from the plane’s ceiling, sections of which had fallen down or been dented and broken.

Debris, including fruit salad, kettles and trays of in-flight meals, was strewn about the cabin, eyewitness footage shows.

The floor was damp with spilled coffee, wine and water, Davies remembered.

Turbulence – or pockets of disturbed air – can have many causes. Singapore Airlines did not say what type was involved.

Weather reports show severe thunderstorms in the area.

“It was cloudy outside, completely white,” Azmir said.

The pilot declared a medical emergency and diverted the plane to Bangkok, landing around an hour later, and was met by a vast number of ambulances and emergency workers.

“Every single cabin crew I saw was injured,” Davies said, expressing gratitude to the staff who continued to help passengers.

The crew were not seated at the time, he noted, as they were serving drinks and performing other duties.

“The crew and people inside lavatories were hurt the most… There were a lot of spinal and head injuries,” Azmir said.

Davies said he helped lay a passenger on the floor who medical professionals on the plane tried unsuccessfully to resuscitate for around 20 minutes.

Emergency crews lifted injured passengers over their heads on stretchers down the narrow aisles, while other passengers remain seated.

As a passenger filmed themselves walking through the carnage to disembark, a voice can be heard saying: “There are still people on the ground.”

Medical tents were set up on the tarmac to examine the injured; some bound to stretchers, some in wheelchairs.

Passengers and crew not being treated in Thai hospitals were ferried to Singapore on another flight early on Wednesday morning, and were met on arrival by Singapore Airlines CEO Goh Choon Phong.

“I was very conscious of every bump on that flight and I made absolutely sure that my seatbelt was firmly fastened the whole time,” Davies said.