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 inflight meals and kettles 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," Azmir told Reuters.

One passenger died of a suspected heart attack and at least 30 were injured after the flight encountered what the airline described as sudden extreme turbulence around 10 hours into the journey as it flew over the southern tip of Myanmar.

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 entire thing happened within the span of less than 10 seconds," Azmir said. "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."

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.

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.

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

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 early Wednesday morning, met on arrival by Singapore Airlines CEO Goh Choon Phong.

"No, we were very lucky," said one passenger on arrival in Singapore when asked if they were hurt.