Two marine police officers were assaulted and pushed into the sea early yesterday morning as they attempted to board a sampan that was allegedly trespassing in Deep Bay, which borders Shenzhen.
Their assailants then swam to a nearby sampan before escaping.
One of the officers who fell into the sea lost his baton and helmet, which have yet to be recovered.
An identification document issued by mainland authorities, which police believe belongs to one of the suspects, was found at the scene.
During a regular patrol of the marine border at about 2.15am yesterday morning, a sampan was found to have crossed the border into Hong Kong and marine police officers were dispatched to investigate.
Police said the two officers were assaulted and pushed into the sea by two men as the officers attempted to board the vessel.
The two assailants then jumped into the sea and swam to another sampan nearby on the Shenzhen side and fled.
The two officers suffered no injuries and were rescued by fellow officers on the same police vessel.
It is not known whether the sampans belong to any organised crime groups or why one had sailed into the city's waters, a police spokeswoman said. The abandoned sampan was seized by police.