ONE person on a Singapore Airlines flight died and dozens of others were injured when the plane traveling from London to Singapore "encountered sudden extreme turbulence," the airline said Tuesday.

Flight SQ321 from London's Heathrow Airport was diverted to Bangkok and touched down at 3:45 pm local time Tuesday at Suvarnabhumi Airport, the carrier said in a statement posted to its Facebook page.

"We can confirm that there are injuries and one fatality on board the Boeing 777-300ER," an earlier statement said, adding that there were 211 passengers and 18 crew members on the plane.

The passenger who died was 73-year-old British man Geoffrey Kitchen, from Thornbury, Gloucestershire. He and his wife were on their way to a holiday in Australia via Singapore, reports CBS News.

According to Thai authorities, he had a heart condition and probably had a heart attack. Seven people, including some with head injuries, were critically injured, and dozens of others were hurt with varying injuries, Suvarnabhumi Airport director Kittipong Kittikachorn said at a news conference.

Bangkok's Samitivej Srinakarin Hospital said it treated 71 people in total, including six with severe injuries, news agencies reported.

The airline offered "its deepest condolences to the family of the deceased" and said it was providing assistance to all passengers and crew from the flight.

Boeing also offered condolences to the family of the passenger who died in a statement posted on social media. The US aerospace giant said it was in contact with Singapore Airlines and ready to support the carrier as it looked into the incident.

Tracking data from the FlightAware website showed the Singapore Airlines plane dropping suddenly from an altitude of about 37,000 feet to 31,000 feet in the space of only about five minutes.

The drop came about 10 hours into the flight from London as the Boeing 777 finished crossing the Andaman Sea and approached the Thai coast. Thunderstorms were reported in the area.

"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 seatbelt was launched immediately into the ceiling," passenger Dzafran Azmir told the Reuters news agency.

"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."

Another passenger, Andrew Davies, who spoke with BBC News said there was "very little warning" before the plane plummeted, but that the seatbelt warning light had come on.

"I put on my seatbelt straightaway then the plane just dropped," he said, describing chaos in the cabin as people shouted for a defibrillator and passengers with medical training tried to assist those injured.

"One of the Singapore Airlines crew said it was by far the worst in her 30 years of flying," Davies said in one tweet, adding: "Lesson is - wear a seatbelt at ALL TIMES. Anyone who is injured, was not wearing a seatbelt."