A Chinese man arrested by police with a gun was sentenced to 16 years jail on Friday for a plot to harm veteran Hong Kong democrat Martin Lee and a prominent newspaper publisher.

Huang Nanhua, 50, was found guilty of the charge of "carrying arms and ammunition with intent to commit an arrestable offence", local media reported, as well as the unlicensed possession of firearms.

Local media said Huang had meant to assassinate Lee last August when he was caught during a routine police roadblock soon after his arrival in Hong Kong from the southern Chinese city of Shenzhen. He was carrying a homemade gun, bullets and the contact details of Lee at the time.

The court was told Huang had earlier told police he wanted to "teach someone a lesson", but he pleaded not guilty.

In passing sentence, deputy High Court judge Peter Line said Huang had intended to use violence to harm Lee, RTHK radio reported.

Despite the jail term, Lee, 71, told reporters that he believed a mastermind behind the plot remained at large.

"I don't know this person (Huang) so there's clearly someone behind him, and he's not been exposed yet," he told reporters.

"The police said they'd continue to investigate. I have confidence in them," added Lee, a leading figure in the territory's political and democratic development before and after 1997, when the former British colony reverted to Chinese rule.

Huang also had the contact details of well-known newspaper publisher Jimmy Lai, whose tabloid Apple Daily is known for its pro-democracy leanings and critical stance towards China.

Huang was arrested a month before Hong Kong's legislative council elections, during which Lee and other pro-democracy politicians campaigned hard for Beijing to allow direct elections in the city by 2012, rather than 2017 as now planned.

While Lee and the city's pro-democracy camp have long been a thorn in the side of China's Communist leaders, Lee himself has dismissed the likelihood of Beijing's involvement.

In 2006, another Democrat, Albert Ho, was attacked in a McDonald's restaurant by men wielding baseball bats. Four men were jailed. Police have said the two cases may be linked.