The Singapore Airlines Boeing 777-300ER airplane seen on the tarmac at Suvarnabhumi International Airport in Bangkok on Wednesday. (AFP)

BANGKOK -- A Malaysian woman whose six relatives and a friend were injured on a Singapore Airlines flight hit by deadly turbulence this week said on Friday they had only seconds to react before the plane started to plunge.

One passenger died and more than 100 were injured when the Boeing 777-300ER fell 1,800 meters in just a few minutes during the final hours of its journey from London to Singapore on Tuesday.

Passengers and crew on the flight sustained skull, brain, and spine injuries as they were tossed violently around the cabin.

The flight carrying 211 passengers and 18 crew was forced to make an emergency landing in Bangkok, where at least 48 people are still being treated in hospital.

Eva Khoo, who prayed for her family at the Erawan Shrine in the Bangkok city centre on Friday, said she was desperately concerned for her pregnant sister-in-law. "My sister-in-law had to have surgery on her spine," she told AFP. "I am really worried because she is pregnant."

The family was returning from a two-week holiday in Switzerland and Britain.

Khoo said her brother was still in pain after the high-altitude ordeal. "His hand is in pain and still numb," she said. "He couldn't carry anything. He still needs a wheelchair to move him around." She said four of her relatives who were on the flight were treated in intensive care at a Bangkok hospital. "Some of them are still in bed and can't be moved," she said.

Her brother told her that the seatbelt sign was off and the situation was "very calm" when the turbulence hit.

"They were serving food and collecting rubbish and suddenly the plane was shaking and my brother and his wife were sitting without (a) seatbelt," she said. "They felt the vibration and the shaking, they wanted to fasten the seatbelt, but (they) only (had) a few seconds." Her brother "flew away" and hit the floor while he was trying to fasten his belt, she said.

Her sister-in-law flew up and dropped onto a seat in the plane.

She said a family friend who was on the flight had fastened his seatbelt but still sustained a neck injury and will have to use back support for at least six months.

- Turbulence danger -

The plane was met at Bangkok's Suvarnabhumi Airport by emergency responders who used gurneys to ferry the injured to ambulances waiting on the tarmac.

Photos taken inside the plane after it landed in Bangkok show the cabin in chaos, strewn with food, drinks and luggage, and with oxygen masks dangling from the ceiling.

Singapore Airlines chief executive Goh Choon Phong has apologised for the "traumatic experience" and expressed condolences to the family of the deceased -- a 73-year-old British man.

The carrier said on Friday it has tightened seatbelt rules on its flights after the incident and that it has introduced a "more cautious approach" to turbulence.

Investigators are analysing cockpit data -- including the voice recorder -- as they seek to understand the cause of the deadly incident.

Air safety experts have told AFP that passengers are often too casual about wearing seatbelts, leaving them at risk if the plane hits unexpected turbulence.

Scientists also warn that so-called clear air turbulence, which is invisible to radar, is getting worse because of climate change. (AFP)