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.

The 78-year-old's sentence is by far the harshest handed out under the Beijing-imposed national security law, surpassing the previous record of 10 years given to legal scholar Benny Tai in 2024.

A summary document from the judges said they had settled on 20 years' imprisonment "after considering the serious and grave criminal conduct of Lai".

Two of those years will overlap with Lai's existing prison term, meaning that he will serve an additional 18 years, the judges wrote.

Lai, who has been behind bars since 2020, sat impassively in the dock as his sentence was read out, an AFP journalist in the court saw.

As he was led away, he waved solemnly to people in the public gallery, including his wife Teresa, former Hong Kong bishop Cardinal Joseph Zen, and former Apple Daily reporters.

His defence lawyer Robert Pang declined to comment on whether Lai would lodge an appeal, which can be done in the next 28 days.

Asked if the sentence was in line with expectations, Pang told AFP: "In these times, I don't know what to expect."

Teresa Lai was grim-faced during the hearing and made no comment as she left the court, but their children, who live abroad, condemned the sentence in a statement.

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

The mogul's daughter Claire called it "a heartbreakingly cruel sentence" given his declining health in prison, and if carried out, "he will die a martyr behind bars".

- 'Effectively a death sentence' -

Rights groups also said Lai's punishment amounted to a grave injustice.

"The harsh 20-year sentence against 78-year-old Jimmy Lai is effectively a death sentence. A sentence of this magnitude is both cruel and profoundly unjust," Elaine Pearson, Asia director at Human Rights Watch, said in a statement.

Amnesty International called the case "another grim milestone in Hong Kong's transformation from a city governed by the rule of law to one ruled by fear".

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".

British Prime Minister Keir Starmer previously said he had raised the issue of Lai during his meeting with Chinese leader Xi Jinping in Beijing last month, while US President Donald Trump has also called for his release.

Following the sentencing, British Foreign Secretary Yvette Cooper called on Hong Kong to "end his appalling ordeal" and pledged to "rapidly engage further" with Beijing.

Taiwan said Lai's sentencing created a "chilling effect" across borders and "tramples on freedom of speech".

Beijing has dismissed critics as smearing Hong Kong's judicial system, while Hong Kong authorities say Lai's case "has nothing to do with freedom of speech and of the press".

Hong Kong's chief executive John Lee welcomed the sentencing on Monday.

"Jimmy Lai's crimes are heinous and evil in the extreme. His heavy sentence of 20 years' imprisonment demonstrates the rule of law, upholds justice and is deeply gratifying," Lee said in a statement.

- Staunch supporters -

Dozens of former Apple Daily employees and others braved the cold to queue overnight outside the West Kowloon court for a spot in court, while journalists gathered outside the building's entrance.

The mood was tense as police deployed dozens of officers, an armoured car and a bomb disposal van to the area.

"I think this is equivalent to sentencing Lai to life imprisonment," Lam Ying-kit, a former history teacher in his 50s, told AFP.

Lai's supporters, children, lawyers and rights groups have all raised concerns about his deteriorating health in prison.

Lai was kept in solitary confinement at his own request to avoid harassment, according to prosecutors, while authorities maintain he receives "adequate" care.

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.

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

Eight other defendants, including six Apple Daily executives, were handed sentences 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.