A man has died and seven people are critical after a Singapore Airlines plane plunged nearly 2000 metres during meal service, with passengers slamming their heads into the ceiling and overhead compartments.

The planet hit severe turbulence about 10 hours into its journey from London to Singapore, forcing the pilot to make an emergency landing in Bangkok, Thailand overnight Tuesday (AEST).

Fifty-six Australians were on board the terrifying flight – the largest nationality among the passengers. Thirty people were injured, and 18 hospitalised.

Seven people were critically injured with head injuries.

Passengers not wearing seatbelts reportedly “shot up” and hit their heads on the ceiling when the plane suddenly plunged.

The Boeing 777-300ER jet fell into an air pocket while the cabin crew was serving breakfast before it encountered what has been termed “sudden extreme turbulence”.

A 73-year-old man from Britain, identified as Geoff Kitchen, died during the incident, likely due to a heart attack, according to officials.

Chaos on board Singapore Airlines SQ321. Photo: X

Tracking data captured by FlightRadar24 and analysed by the Associated Press shows the Singapore Airlines flight SQ321 cruising at an altitude of 11,300 metres.

At one point, the plane suddenly and sharply plummets to 9400 metres in three minutes, the data shows. The aircraft stayed at 9400 metres for less than 10 minutes before diverting and landing in Bangkok within half an hour.

The sharp descent in turbulence happened as the flight was over the Andaman Sea, near Myanmar.

The aircraft sent a “squawk code” of 7700, an international emergency signal.

‘Dramatic drop’

One passenger on the flight told Reuters that the incident involved the sensation of rising then falling.

“Suddenly the aircraft starts tilting up and there was shaking. So I started bracing for what was happening, and very suddenly there was a very dramatic drop, so everyone seated and not wearing a seatbelt was launched immediately into the ceiling,” Dzafran Azmir, a 28-year-old student on board, told Reuters.

“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,” he said.

Kittikachorn said most passengers he had spoken to were wearing their seatbelts.

The plane with 211 passengers and 18 crew was headed to Singapore when it made the emergency landing, the airline said.

Singapore news outlet CNA carried blurry pictures supplied by readers that it said appeared to be from the flight.

They showed anxious passengers clinging to seats, with oxygen masks hanging from above, personal items strewn across the aisle and rubbish spilled on the floor of the cabin crew area.

Suvarnabhumi airport said the plane requested an emergency landing at 3.35pm local time and landed at 3.51pm.

Uninjured passengers disembarked and an another aircraft will fly them onwards.

Singapore Airlines, which is widely recognised as one of world’s leading airlines and is a benchmark for much of the industry, has had no major incidents in recent years.

Terrified passengers strap themselves in. Photo: X

Its last accident resulting in casualties was a flight from Singapore to Los Angeles via Taipei, on October 31, 2000. It crashed into construction equipment at the Taiwan Taoyuan International Airport after attempting to take off from the wrong runway.

The crash killed 83 of the 179 people on board.

Singapore Airlines has had seven accidents, according to records from the Aviation Safety Network.

Boeing said it was in touch with Singapore Airlines and was ready to provide support. It referred further questions to the airline and local authorities.

“We extend our deepest condolences to the family who lost a loved one, and our thoughts are with the passengers and crew,” it said.

-with AAP