Police say 43 people were injured after a Kowloon Motor Bus double-decker bus crashed into the back of a crane truck in Tuen Mun on Sunday.
Police said the truck had parked on the side of Tuen Mun Road at around 1pm due to suspected mechanical problems.
One bus passenger suffered serious head and neck injuries, while five others suffered broken limbs or facial injuries.
The driver of the bus had to be freed before being taken to hospital.
More than 100 firefighters and paramedics were involved in the rescue operation and the injured were taken to four different hospitals.
Officers said they were investigating the crash, including the condition of the bus driver.
"We believe the truck driver thought there might be a mechanical problem with the vehicle, so he pulled over to the hard shoulder so he could call for assistance," said New Territories North Traffic Superintendent Lam Chi-yuen.
"According to the information we have at this stage of our investigation, the vehicle's hazard lights were on at the time."
The bus company said it would cooperate with the police investigation.
The 58M bus had been en route to Kwai Fong Station from Leung King Estate in Tuen Mun.