More than 140 passengers and crew from a Singapore Airlines flight on which one person died during heavy turbulence finally reached Singapore on Wednesday, with 20 still fighting for their lives in a Bangkok hospital.

The London-Singapore flight on a Boeing 777-300ER plane diverted to Bangkok for an emergency landing on Tuesday after the plane was buffeted by turbulence that flung passengers and crew around the cabin, slamming some into the ceiling.

A 73-year-old British passenger died of a suspected heart attack, according to an airport official, and Bangkok's Samitivej Hospital said 20 passengers were in intensive care, nine had undergone surgery, while five more were awaiting surgery.

Josh Silverstone, from south London, woke up on the floor of the plane.

"I (...) didn't realise what happened. I must have got hit in the head somewhere. Lots of people hit their head. Everyone was bleeding," he told Reuters as he left the hospital on Wednesday evening after being treated. He had a cut on his eye.

He had been heading to Singapore for a holiday before meeting friends in Bali. He said he still hopes to make it to the Indonesian island.

Many passengers in the hospital still being treated had spinal injuries.

"I am lucky to be able to walk," he said.

Photographs of the interior of the plane showed gashes in the overhead cabin panels, oxygen masks and panels hanging from the ceiling and luggage strewn around. A passenger said some people's heads had slammed into the lights above the seats and broken the panels.

"I saw people from across the aisle going completely horizontal, hitting the ceiling and landing back down in like really awkward positions. People, like, getting massive gashes in the head, concussions," Dzafran Azmir, a 28-year-old student onboard the flight, told Reuters after arriving in Singapore.

Singapore Airlines flew 131 passengers and 12 crew on the relief flight from Bangkok, which reached Singapore just before 2100 GMT. There were 211 passengers including many Australians, British and Singaporeans, and 18 crew on the original flight. Injured fliers and their families remained in Bangkok.

"On behalf of Singapore Airlines, I would like to express my deepest condolences to the family and loved ones of the deceased," Singapore Airlines CEO Goh Choon Phong said in a video message.