Footage from the scene showed rescue workers cutting open the roof of the bus, operated by the Kowloon Motor Bus Co.
Another passenger said: "He was driving very fast, extremely fast, even if he was driving down a slope". Though there is no confirmation about the cause of the accident, reports said the vehicle was speeding.
Among those, according to the Times, 15 were hospitalized in serious condition, and 10 were deemed critical.
One injured passenger told the Oriental Daily said it was like the driver was "intentionally using the bus to throw a tantrum".
The injured passenger told the paper: "It was much faster than I normally felt in a bus". "And then it was like the tyre slipped, and the bus turned".
A full-scale response from the emergency services saw dozens of police, firefighters and paramedics race to the scene to disentangle victims from the battered bus.More news: New iOS 11.3 Feature Slow Down Your Processors - Turn It Off!
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City leaders and the bus company will carry out separate investigations into the crash.
Rescuers work at the accident site where a double-decker bus overturned in New Territories in Hong Kong Special Administrative Region, south China, Feb. 10, 2018.
Saturday's accident appears to be the worst since July 2003, when a double-decker fell off an elevated road after being hit by a lorry, killing 21 people.
KMB general manager Godwin So Wai-kei said the 30-year-old unidentified driver joined the company in 2014 and had driven the route before.
Last September, three people were killed and 31 injured after a City bus double-decker mounted a kerb in Sham Shui Po.