A 73-year-old man from Britain died and seven people were critically injured after a plane encountered “sudden extreme turbulence” about 10 hours into a flight from London to Singapore, officials said on Tuesday.

The plane, a Boeing 777-300ER operated by Singapore Airlines, was diverted to Bangkok, the airline said in a statement, and landed at 3:45 p.m. local time on Tuesday.

In all, 30 people, including passengers and crew members, were injured, officials said. The airline said in its statement that 18 people had been hospitalized and another 12 people were being treated for injuries. “The remaining passengers and crew are being examined and given treatment, where necessary, at Suvarnabhumi International Airport in Bangkok,” the airline said.

The man died aboard the flight, said Kittipong Kittikachorn, the director of Bangkok’s airport, without identifying him by name or giving a cause of death. The man’s wife was injured and taken to a hospital, Mr. Kittipong said.

The flight, which had 211 passengers and 18 crew members on board, had left Heathrow Airport on Monday.

The average flight time of that route is just under 13 hours. Mr. Kittipong said at a news conference that he went onto the airplane after it landed and described the scene as “a mess.”

The flight, SQ321, took off from Heathrow at 10:38 p.m. local time on Monday. The airline said it encountered turbulence over the over the Irrawaddy Basin over Myanmar, at 37,000 feet. Data on Flight Radar 24, a website that compiles public information about flights, appears to show that about 10 hours later, it went from 37,000 feet to an altitude of roughly 31,000 feet in only a few minutes.

The pilot declared a medical emergency and diverted the flight to Bangkok.

About 100 passengers who were not injured were to be flown to Singapore on Tuesday, Mr. Kittipong said. He described them as being in a state of shock.

It’s unclear what caused the episode. As the plane traveled across Myanmar, satellite data showed a strong storm beginning to form and bubble into the higher elevations, which suggests that the atmosphere in the region was becoming unstable. The plane was also moving toward other storms that were developing along the coast of Myanmar.

Deaths caused by turbulence are rare. Between 2009 and 2021, 146 passengers and crew members were seriously injured in turbulence, according to data from the Federal Aviation Administration. In December 2022, 11 people were seriously hurt in turbulence on a flight from Phoenix to Honolulu.

José Alvarado, a pilot with the Icelandic airline Play, said that in his experience, clear-air turbulence, which happens most frequently at high altitudes, can occur without warning. For that reason, he tells passengers, “even if there’s no turbulence, keep your seatbelts on.”

Once, more than two decades ago, when he was working as a flight attendant, he experienced turbulence on a flight from Madrid to Buenos Aires that was so severe that he was thrown upward when the plane suddenly dropped about 4,000 feet. His back and shoulder blades hit the ceiling of the cabin before he was thrown back down.

“I was just bouncing up and down,” he said, adding that some passengers also were injured. He said he had not experienced anything like that since.

Chee Hong Tat, Singapore’s minister for transport, said in a statement that he was “deeply saddened to learn about the incident.”

Singapore Airlines offered its condolences to the family of the person who died on the flight, adding that “we deeply apologize for the traumatic experience that our passengers and crew members suffered on this flight.”

Judson Jones and Jenny Gross contributed reporting.