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

The 78-year-old's sentence is by far the harshest under Hong Kong's national security law, which Beijing imposed in 2020 after huge pro-democracy protests in the former British colony.

The three High Court judges said in a summary they had settled on 20 years' imprisonment "after considering the serious and grave criminal conduct of Lai".

Lai, who has been behind bars since 2020, sat impassively in the dock as his sentence was read out. He waved to family members and supporters in the public gallery as he was led away.

His wife Teresa was grim-faced and made no comment as she left the court, but two of his children based abroad condemned the sentence.

"Sentencing my father to this draconian prison sentence is devastating for our family and life-threatening for my father," Lai's son Sebastien said in a statement.

The mogul's daughter Claire called it "a heartbreakingly cruel sentence" which, because of his deteriorating health, effectively meant "he will die a martyr behind bars".

Lai has 28 days to lodge an appeal, but his lawyer declined to say if he would do so.

The United States criticised the decision as "unjust and tragic," and reiterated its call for Lai to be granted humanitarian parole.

The ruling "shows the world that Beijing will go to extraordinary lengths to silence those who advocate fundamental freedoms," said US Secretary of State Marco Rubio in a statement.

- 'Appalling ordeal' -

Hong Kong's leader John Lee lauded the "severe" prison term, saying it "brings great relief to all" and demonstrated how the city upheld the rule of law.

Beijing also praised the outcome as "reasonable, legitimate and legal".

The commissioner of China's foreign ministry in Hong Kong wrote to foreign media outlets, including AFP, on Monday and urged them to "report on the case with objectivity and fairness, refraining from politicising legal matters".

Washington's reaction added to a chorus of international condemnation, including from the European Union, Australia, Japan, and advocacy groups.

"This verdict needs to be promptly quashed as incompatible with international law," UN High Commissioner for Human Rights Volker Turk said in a statement that described the national security law as "vague and overly broad".

Human Rights Watch said the penalty was "effectively a death sentence" given Lai's age, calling it "cruel and profoundly unjust".

Jodie Ginsberg, the CEO of the Committee to Protect Journalists, said "today's egregious decision is the final nail in the coffin for freedom of the press in Hong Kong".

Hong Kong most recently ranked 140th in the world for press freedom, down from 70th a decade ago, according to Reporters Without Borders.

- Health issues -

The three High Court judges described Lai, long a thorn in Beijing's side, as the "mastermind and the driving force" behind the conspiracies.

Two years of the 20-year penalty overlap with an existing sentence, meaning he will effectively serve another 18 years in jails. Lai would then be 96 years old.

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

Lai has been kept in solitary confinement at his own request to avoid harassment, according to prosecutors.

His supporters, children, lawyers and rights groups have all raised concerns about his deteriorating health in prison, although authorities maintain he receives "adequate" care.

Eric Lai, a senior fellow with the Georgetown Center for Asian Law, said Hong Kong's legal system had become a tool for political control.

"After the regime's sustained effort to portray Lai as an 'enemy of the state', a reversal from the appellant court would contradict years of political theatre," said Lai, who is unrelated to the mogul.

Former Apple Daily reporter Tammy Cheung told AFP outside the court that she feared she "may never see Lai in person again".

"Even before this sentencing today, the industry was constantly self-censoring. Now after this sentencing, I think the impact it will have on this industry is quite clear for all to see," she said.

Retiree Lai So said there are "fewer reports on the negative side of society" since Lai's once-popular Apple Daily was shut down in 2021.

"The space for freedom of speech has shrunk a lot," So said.