(Reuters) - There was little warning of the chaos that was about to be unleashed onboard Singapore Airlines flight SQ321.

With around three hours left on the journey from London to Singapore, Malaysian student Dzafran Azmir got the uneasy feeling the Boeing 777-300R plane was tilting upwards and beginning to shake.

The 28-year-old braced himself and checked he had his seatbelt on. He did. Many of the other passengers did not, he said.

"Suddenly there was a very dramatic drop so everyone seated and not wearing seatbelt was launched immediately into the ceiling, 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," Azmir told Reuters.

"People dropped to the ground, my phone flew out of my hand and went a couple aisles to the side, people's shoes flung about," he added.

One passenger was killed and 30 injured after the flight from London fell into an air pocket before encountering turbulence en route on Tuesday, forcing it to make an emergency landing in Bangkok, officials and the airline said.

"The crew and people inside lavatories were hurt the most because we discovered people just on the ground not able to get up. There were a lot of spinal and head injuries," Azmir said.

The captain informed passengers they would be making an emergency landing in Thailand's capital Bangkok.

Once the plane was on the tarmac, nurses and rescue workers came in to check on the injured, Azmir said.

"I don't think they anticipated how bad it was," he said.

Ambulances later arrived and Azmir said he saw at least 8 people on stretchers being pulled out of the emergency exits. It took 90 minutes to evacuate the plane, he said.

(Reporting by Timour Azhari; writing by Joe Brock; editing by Alexandra Hudson)