The flight from London and bound for Singapore fell into an air pocket while cabin crew were serving breakfast before it encountered turbulence, prompting the pilots to request an emergency landing, Bangkok Suvarnabhumi airport general manager Kittipong Kittikachorn told a press conference.
The sudden turbulence occurred over the Irrawaddy Basin in Myanmar about 10 hours into the flight, the airline said.
The pilot declared a medical emergency and diverted the aircraft to Bangkok, it said without giving further details.
Photographs from the interior of the plane showed large gashes in the overhead cabin panels, gas masks and panels hanging from the ceiling and items of hand luggage strewn around.
A passenger said some people's heads had slammed into the lights above the seats and punctured the panels.
"I saw things lying everywhere and many air crew injured" with bruising, Kittikachorn said after the most critically injured passengers and crew had been removed from the plane.
A 73-year-old man from the United Kingdom died during the incident, likely due to a heart attack, Kittikachorn said.
Seven people were critically injured, some with head injuries.
He added people were calm as they were led from the plane.
"Singapore Airlines offers its deepest condolences to the family of the deceased. We deeply apologise for the traumatic experience that our passengers and crew members suffered on this flight," the airline said.
Some tallies of the injured out of the 211 passengers and 18 crew differed.
The airline said 18 were hospitalised and 12 being treated in hospitals.
Samitivej Hospital said it was treating 71 passengers, including six who were severely injured.
Singapore Airlines says 18 people were taken to hospital after their plane landed in Bangkok. (AP PHOTO)
Singapore Airlines said in a social media post that 56 people on board the plane were from Australia - the largest group - while 47 were from the UK, 41 were from Singapore and 23 from New Zealand.
Tracking data captured by FlightRadar24 and analysed by the Associated Press show the Singapore Airlines flight SQ321 cruising at an altitude of 11,300 metres.
At one point, the Boeing 777-300ER suddenly and sharply plummets to 9400 metres over the span of three minutes, the data shows.
The aircraft then stayed at 9400 metres for under 10 minutes before diverting and landing in Bangkok less than half an hour later.
The sharp descent in turbulence happened as the flight was over the Andaman Sea, near Myanmar.
The aircraft sent a "squawk code" of 7700 at that time, an international emergency signal.
"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 the flight 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 of the passengers he had spoken to had been wearing their seatbelts.
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 not had any major incidents in recent years.
Its last accident resulting in casualties was a flight from Singapore to Los Angeles via Taipei, where it crashed on October 31, 2000 into construction equipment on 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 by 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 AP
