Photographs on local media websites showed the bus lying on its side with its roof torn off by a road near the town of Tai Po in the northern New Territories.
It appeared to have slid on its side and hit a lamppost which cut through the front of the vehicle.
Rescuers were attempting to reach passengers trapped in the wreckage, although police were unable to say how many were still inside the bus.
The dead included 17 males and two females, police said.
It was not immediately known what caused the accident but passengers quoted by local media said the driver was going fast before the crash.
"It was much faster than I normally felt in a bus," an 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," he said.
Passengers had complained to the driver who was reportedly 10 minutes late and he then kept speeding up before the bus crashed, the Apple Daily reported, quoting injured passengers at the scene.
Lawmaker Lam Cheuk-ting of the Democratic Party urged the government to investigate bus designs, "especially with the upper deck of buses having been repeatedly scrapped off in accidents, posing a serious threat to passengers on the upper level".
The southern Chinese city promotes its public transport system as one of the best infrastructures 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.
Fourteen people were injured last April when a double-decker tram tipped over, with a 23-year-old driver later arrested for dangerous driving causing harm to others.
The safety of Hong Kong's waterways has also remained under scrutiny since 39 people were killed when a high-speed ferry and a pleasure boat crashed in 2012.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)