The double-decker bus overturned Saturday evening near the town of Tai Po in the northern New Territories, flipping onto its side and appearing to smash into a lamppost.
Nineteen people were killed and 65 people were injured, some critically, according to local police.
"The 30-year old male bus driver was arrested for dangerous driving causing death and dangerous driving causing grievous bodily harm. He is still being detained for further enquiries," police said in a statement early Sunday.
Most of the injured and some of the dead were on the upper deck of the bus, Chan Hing-yu of the fire department told reporters.
The driver was suspected of being over the speed limit as he went down a slope and lost control of the vehicle, senior traffic superintendent Lee Chi-wai told reporters.
He was not in need of any medical treatment after the crash and was found to be sober, he added.
Long hours, low pay
The accident has reignited a debate over working conditions for the city's bus drivers.
The vehicle was managed by the Kowloon Motor Bus Company, one of the main bus operators in Hong Kong.
Its "management is at fault, and it did not attach importance to traffic safety nor to the staffing structure, work and rest, and training of drivers," Lai Siu-chung, a representative of the motor transport workers union branch at the company, told reporters Sunday, according to local broadcaster TVB.
Lai said the company's poor treatment of workers had led to labour shortages, adding that many drivers work under pressure and without adequate support.
"The industry wages of drivers have lagged behind inflation for many years... as a result the number of drivers working extra shifts and part-time have increased," said lawmaker Luk Chung-hung, who also questioned whether the company was paying enough attention to safety.
The Kowloon Motor Bus Company said it would pay compensation to survivors and victims' families, but has not specifically responded to these allegations.
'Serious threat'
Speaking to local media, passengers said the bus was going too fast before the crash.
"It was much faster than I normally felt in a bus," one injured passenger told the South China Morning Post's online edition.
"And then it was like the tyre slipped, and the bus turned. It was really chaotic in the bus. People fell on one another and got tossed from side to side."
Before the crash, passengers had complained to the driver who was reportedly 10 minutes late and he then started speeding up, the Apple Daily reported, quoting injured passengers at the scene.
One injured passenger told the Oriental Daily it was like the driver was "intentionally using the bus to throw a tantrum."
Lawmaker Lam Cheuk-ting of the Democratic Party urged the government to rethink the design of double-decker buses, saying the upper decks had been "repeatedly torn off in accidents, posing a serious threat to passengers on the upper level."
He also called on the government to address the issue of many drivers working overly long hours.
City leader Carrie Lam, who visited survivors at the Prince of Wales Hospital late on Saturday, expressed "deep sorrow" and pledged there would be an independent investigation.
The southern Chinese city promotes its public transport system as one of the best in the world but fatal accidents do occasionally happen.
Hong Kong's worst road traffic accident occurred in 2003 when a double-decker bus collided with a truck and plunged from a bridge, killing 21 people.
In 2008, 18 people were killed in another bus crash.
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