Chum Tai-kong, the Ma On Shan fire station commander, said a technician had been carrying out work in the restaurant's air conditioning machine room at the time of the blast.

"We are investigating what kind of work he was doing and what equipment he used," Chum said.

Nine men and 12 women, aged between seven and 92, were injured or reported feeling unwell after the blast.

The technician and a man who was trying to help his mother-in-law escape suffered burns to their faces and limbs.

Witnesses reported hearing an explosion, then seeing smoke in the dining area, before a fireball burst out of the machine room to the street.

The 2,000 sq ft restaurant was packed with lunchtime diners when the explosion happened at about 12.15pm.

A female employee said: "I first saw smoke and then shouted for diners to run. As I reached the exit, an explosion went off."

An elderly woman, who was having lunch at the restaurant, said: "I heard a loud explosion before smoke poured out from a room. People started running and were in a panic.

"After I ran out of the restaurant, windows were smashed and glass fragments littered the road."

Another woman said some of her friends were pushed to the floor during their escape from the packed restaurant.

A 92-year-old wheelchair user, her daughter, 52, and son-in-law, 59, were seriously hit by the force of the blast. They had been seated at a table near the door of the machine room.

A diner who has lived in the estate for 25 years said the man often took responsibility for pushing his wheelchair-bound mother-in-law.

He failed to make a speedy escape after the accident as he was trying to help her out, the diner said.

"The fireball hit him and his whole upper torso reddened," she said. "I could feel the heat."

A fleet of 15 fire engines and 11 ambulances were put into action and about 120 firefighters and paramedics were deployed.

About 200 people were evacuated from the two-storey commercial complex, police said.

Initial checks by the fire service investigators indicated there was one explosion that came from the 150 sq ft air conditioning machine room, the Fire Services Department said.

A police spokeswoman said a machine caught fire in the room after the blast. Firefighters doused the flames at 1.09pm.

Most of the injured complained of feeling unwell from smoke inhalation or were wounded by flying glass fragments. They were treated at Prince of Wales Hospital in Sha Tin and Alice Ho Miu Ling Nethersole Hospital in Tai Po.

Two of the injured were in a serious condition last night, while the others were in a stable condition or had been discharged.

The Fire Services, Electrical and Mechanical Services, and Labour departments are investigating the cause of the incident.