Photos and videos published by social media showed the gold-coloured bus lying on its side with its roof torn off, while emergency workers treated injured passengers nearby.
It appeared to have slid on its side and hit a lamppost which smashed into the upper decks of the vehicle.
"There were no signs that he was exhausted" on his four-hour shift Saturday, So said. The driver has been arrested on charges of causing death and grievous bodily harm by risky driving.
Authorities say dozens more are injured after the bus flipped over in the southern Chinese city.
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According to injured passengers, the bus was allegedly going faster than allowed after they had complained that the bus was 10 minutes behind schedule, local media reported.
According to the South China Morning Post, fifteen men and three women were confirmed dead at the scene, while 61 injured passengers were rushed to 12 public hospitals across the city. "And then it was like the tyre slipped, and the bus turned". It was really chaotic on the bus. "People fell on one another and got tossed from side to side".
General Manager Godwin So Wai-kei attended the scene of the crash, telling reporters the company would issue a "condolence allowance" of HK$80,000 (£7,200, $10,000) to injured passengers and the families of the deceased.
The crash was deadlier than any bus accident in Hong Kong since one in 2003 that killed 21 people.