Passengers were hurled off their seats, some suffering serious injuries, when an SAA flight from South Africa to Hong Kong experienced severe turbulence over Kuala Lumpur early on Wednesday.

It felt like the plane was in free-fall. The pilot had just turned on the "fasten seatbelts" lights, but before he could make the announcement of the upcoming turbulence everyone on the plane was thrust into the air.

Of the 20 hurt, 17 were passengers. The rest were crew members, SAA spokesman Tlali Tlali said.

"Two were seriously injured. The other 18 experienced minor injuries," he said. "The flight crew immediately requested medical assistance for the passengers, which was ready on arrival of the aircraft in Hong Kong," said Tlali.

"The two passengers who sustained serious injuries remain in hospital for observation," Tlali said in a statement.

The four-engine Airbus A340-300 had a very bumpy ride while flying over Kuala Lumpur, the Malaysian capital, after taking off from OR Tambo International Airport on Tuesday evening.

Despite the drama, the plane landed safely in Hong Kong on Wednesday at about 6.30am SA time, (12.30pm Hong Kong time).

Hong Kong's emergency services were on standby to assist the victims and took them to three hospitals. Some passengers were taken off the plane on stretchers.

The South China Morning Post reported that while flying through the turbulence, many of the 165 passengers were lifted off their seats and hit the cabin ceiling, resulting in head and neck injuries.

Witnesses told the paper that many of the passengers were sleeping at the time. Most of the injured were seated at the rear of the plane.

The massive jolt to the aircraft sent Brian Heuer and his wife, Le-Anne into the air, albeit briefly before their bodies hit the plane's ceiling. "I'm surprised the plane didn't crack in half," Heuer said.

There was barely time to realise his own mortality - and question if the plane was going to crash - before the aircraft restabilised.

Heuer, now covered in bruises, then saw the chaotic aftermath - passengers had been thrown to the floor, some groaning with pain and barely able to move, the roof of the plane was covered in dents and his wife had a large lump on her leg.

David Mkumbuje, who bumped and scratched his head, was quoted as saying the turbulence lasted several minutes. "It was like the end of the world. We started to scream, get worried, this thing is going down!"

Sebastian Ncoana told Xolani Gwala of Talk Radio 702 the experience was like travelling over a bump in a car: "You feel like your lungs are going up. But because of the speed of the plane, it's even more violent."

"Five hours before reaching Hong Kong, the plane went downwards. It lost altitude very violently... causing the passengers to hit the roof of the plane with their heads."

He understood the plane had hit an "air pocket" and had lost altitude - "20-30 feet, very suddenly. We all got confused, got flung very violently, hit the roof".

He had injured his neck.

SAA crew on board had been "very supportive to passengers".

But he stressed: "However, it's not a dangerous thing. It's a part of any turbulence you can face.

"It does not affect the outside structure of the plane, it doesn't compromise the safety of the flight."

Passengers said the SAA crew handled the situation very well, stayed calm and assisted the injured and traumatised after the incident.

Terrified passengers took to Twitter to talk about their experiences. Cape Town's Ewlad Sadie posted a picture of a gaping hole in the cabin roof: "Worst flight ever. Crazy turbulence sent people flying everywhere. Guy next to me made this with his head. Lots of passengers with bumps and scratches and a few serious spinal injuries. I'm all good but f****** terrified."

Asanda Ntshiqa from Joburg tweeted: "Made it. Such a terrible experience being stuck in turbulence almost losing my whole family."

Back in South Africa, news of the turbulent trip had made it into the media. Bronwyn Linke received a BBM at about 8.30am from a friend saying: "You're lucky your parents weren't on this flight!" with a link to an article mentioning the flight number.

She immediately recognised that her parents, the Heuers, were on the flight, and began frantically contacting SAA. "I was trying not to panic. I'd heard that the plane had landed safely, but I wanted to make sure (Brian and Le-Anne) weren't hurt," said Linke.

For the next two hours, she waited, trying to keep calm.

"I know it's not on the same scale, but I think I can understand how the families of the missing (Malaysian Airlines plane) felt," said Linke.

Aviation expert and the managing director of PR firm Plane Talking, Linden Birns, said severe turbulence is "quite rare. The problem with severe turbulence is you can't see it until you fly into it, it's invisible to radar. No aircraft is immune from them", he said.

Birns said this type of turbulence was often caused by two pock0ets of air, travelling at widely different speeds, colliding, also known as clear air turbulence (CAT).

Veteran pilot Scully Levin said CAT creates a "whirlpool" effect of winds - which caused the aircraft to shake. The resulting turbulence was rated from light to moderate to severe. The severity of turbulence also depends on the angle the craft enters the whirlpool and the difference in the two wind speeds.

He has never experienced severe turbulence, but said the moderate turbulence he has been through was "horrific".

"With severe turbulence you can battle to see the instruments. But the pilots are also strapped in more securely than passengers. We have five point harnesses; they have two point harnesses."

Another aviation expert said he suspected most of the injured passengers were not strapped to their seats when the plane started shaking.

"When you hit slight turbulence, as a pilot you'll warn the passengers to fasten their seatbelts.

"Unfortunately, a slight shift of wind has the power to send people airborne, even before the pilot can warn them," he said.