Six people were killed and 39 others were injured when a double-decker bus smashed into a tree in Sheung Shui on Wednesday, slicing off much of the vehicle's roof.

The crash happened at about 4 pm on the Fanling Highway near Kwu Tung as the vehicle was on its way from Wan Chai on Hong Kong Island to Sheung Shui in the north-central New Territories.

The police said the 56-year-old bus driver has been arrested on suspicion of dangerous driving causing death. It pledged to launch a "full investigation", looking into the health condition of the driver and his state of mind while driving, as well as the traffic flow and the road at the time of the crash.

Investigations are continuing into the cause of the tragedy, the deadliest traffic accident in Hong Kong since February 2018, when 19 passengers died and 66 others were injured in another bus crash in Tai Po.

Wednesday's accident involved a double-decker operated by Kowloon Motor Bus on Route 978.

The force of the impact was so great that a large part of the vehicle's roof was ripped off, with two passengers on the upper deck thrown out of the bus.

The six who died were three men and three women aged between 31 and 59. Five of them were certified dead at the scene, while the sixth died in a hospital.

According to KMB, the driver has been working in the company for 18 years and has been driving on Route 978 for more than two years. KMB said he had no record of being involved in a traffic accident since 2017, and had passed a body check in September. The bus itself is 5 years old with a valid certificate.

"Shocked and saddened" by the tragedy, Hong Kong Chief Executive Carrie Lam Cheng Yuet-ngor visited the injured in North District Hospital and offered her condolences and sympathies to the victims and their families.

Lam said she has instructed the Social Welfare Department, the Home Affairs Department and the Hospital Authority to provide all necessary assistance.

The injured included two children, 3 and 12 years old. Six of the injured were in serious condition on Wednesday evening, while the rest were either stable or had been discharged.

Lam said the SAR government will make every effort to ensure that the city's bus services are safe.

Acting on Lam's instructions, five interdepartmental desks across the New Territories were set up to render assistance to, and handle inquiries from, the public. The police also set up an inquiry hotline concerning the latest tragedy.

The Transport Department expressed deep concern over the crash, saying it has asked KMB to submit a report and follow up on the case.

Central government authorities on Hong Kong affairs — the Hong Kong and Macao Affairs Office of the State Council, and the Liaison Office of the Central People's Government in the Hong Kong Special Administrative Region — also offered their condolences over the tragedy. The Liaison Office pledged all possible assistance if requested by the SAR government.

KMB said it had sent staff to the scene of the accident and to hospitals to gather more information and cooperate with the police probe.

The company expressed its deep condolences and sympathies to the victims and their families, and pledged to provide them with any assistance necessary.