HONG KONG - Thirty-eight people including five children were killed after two vessels collided on Monday evening off the Lamma Island on the southwest of the Hong Kong Island, and six crew from both vessels have been detained, the Hong Kong Special Administrative Region (HKSAR) government said Tuesday evening.

According to the government, the six individuals detained were under investigation for "endangering people's lives at sea".

A territory-wide condolence will be held on October 4. Regional flags at government buildings will be at half-mast for three days starting Thursday, during which consolatory areas will be set up in all 18 districts of Hong Kong for members of the general public to grieve over the dead.

The incident occurred at the seafront of the Lamma Island, the third largest island in Hong Kong, at about 8:23 pm Monday, when Hong Kong residents were on a public holiday.

One vessel, which sank quickly after the collision, carried over 120 passengers and crew. It was rented by the Hong Kong Electric Company to send some of its staff and their family members for an evening cruise to watch a massive fireworks display at the Victoria Harbor.

The other vessel, which was damaged but did not capsize, was a regular ferry operated by the Hong Kong and Kowloon Ferry Company. It also carried a number of passengers.

"This is definitely an isolated incident. The marine territory of Hong Kong is safe," said CY Leung, chief executive of the HKSAR, a press conference following an inter-departmental meeting.

Rescuers approach a partially-submerged boat after two vessels collided in Hong Kong October 1, 2012.  [Photo/Agencies]