Lai, a British citizen and founder of the now-defunct Apple Daily newspaper, was found guilty in December of urging foreign countries to impose sanctions on Hong Kong and for publishing "seditious" articles in his paper.

On Monday, a Hong Kong court handed down the harshest penalty yet under a Beijing-imposed national security law.

"Jimmy Lai is a publisher sentenced to 20 years in prison for exercising rights protected under international law," UN High Commissioner for Human Rights Volker Turk said in a statement.

"This outcome highlights how the vague and overly broad provisions of Hong Kong's national security legislation can lead to being interpreted and enforced in violation of Hong Kong's international human rights obligations," he warned.

"This verdict needs to be promptly quashed as incompatible with international law."

Rights groups have condemned 78-year-old Lai's punishment as "effectively a death sentence" and a symbol of the city's shrivelling press freedoms.

Turk appealed Monday "for Jimmy Lai's immediate release on humanitarian grounds, given his advanced age, health, and the impact of the more than four years he has already spent in detention".

- 'Broader repressive trend' -

Long a thorn in Beijing's side, Lai was prosecuted under the national security law imposed by China on Hong Kong in 2020 following huge pro-democracy protests.

The UN rights office pointed out that press freedoms had "sharply deteriorated" in Hong Kong since the law took effect, with many independent media outlets closed, and dozens of journalists arrested.

"This is part of a broader repressive trend in Hong Kong," Turk said.

Apple Daily itself was forced to close in 2021 after police raids.

Eight other defendants, including six Apple Daily executives, were handed sentences Monday of up to 10 years in jail. All had pleaded guilty.

Hong Kong had arrested a total of 386 people for various national security crimes by the start of this month, with 176 of them being convicted.

Turk reiterated his call for "the immediate release of those arbitrarily detained under these laws and for their repeal or amendment in line with international standards".

He stressed that he was ready to "engage constructively with the Hong Kong authorities, with a view to bringing the legislative framework into conformity with international law".