The 78-year-old founder of the now-defunct Apple Daily newspaper has been behind bars since 2020.

Here are some key reactions:

- Lai's children: 'Cruel' -

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

His daughter Claire added: "This is a heartbreakingly cruel sentence. Over the last five years, I have watched my father's health deteriorate dramatically and the conditions he's kept in go from bad to worse. If this sentence is carried out, he will die a martyr behind bars."

- Hong Kong leader: 'Deeply gratifying' -

Hong Kong leader John Lee said Lai's crimes were "heinous and evil in the extreme".

"(Lai's) heavy sentence of 20 years' imprisonment demonstrates the rule of law, upholds justice and is deeply gratifying," Lee said in a statement.

- Beijing: 'No room for argument' -

Lin Jian, spokesperson of China's foreign ministry, said Lai was a Chinese citizen and was "a key planner and participant in a series of anti-China and disruptive activities in Hong Kong".

"This is reasonable, legitimate, and legal; there is no room for argument," Lin said at a news briefing.

- UK: 'Rapidly engage further' -

Britain's foreign minister Yvette Cooper said Lai's penalty was "tantamount to a life sentence" and reiterated calls for the tycoon's release on humanitarian grounds.

"Following today's sentencing we will rapidly engage further on Mr Lai's case. We stand with the people of Hong Kong," Cooper added.

- EU: 'Deplores' -

The European Union said it "deplores" Lai's sentencing and called for his "immediate and unconditional release".

"The EU calls on the Hong Kong authorities to restore confidence in press freedom in Hong Kong... and to stop prosecuting journalists," EU foreign affairs spokesperson Anitta Hipper said.

- Human Rights Watch: 'Effectively a death sentence' -

Elaine Pearson, Asia director at Human Rights Watch, said Lai's penalty was "effectively a death sentence".

"A sentence of this magnitude is both cruel and profoundly unjust," Pearson said in a statement.

- Taiwan: 'Chilling effect' -

Taiwan's Mainland Affairs Council said in a statement that China and Hong Kong "have brought media influence and international connections within the scope of national security measures, aiming to create a chilling effect across sectors and borders".

- Reporters Without Borders: 'Collapse of press freedom' -

Thibaut Bruttin, director general of Reporters Without Borders, said Lai's case was "nothing more than a sham".

"This court decision underscores the complete collapse of press freedom in Hong Kong and the authorities' profound contempt for independent journalism," Bruttin said.

"Democracies, such as the UK and the US, must stop prioritising the normalisation of relations with China and instead exert pressure on the Chinese regime and Hong Kong authorities to ensure that Jimmy Lai and all other journalists are released from prison," he added.

- Legal scholar: 'On the high side' -

Simon Young, a law professor at the University of Hong Kong, said the parts of Lai's sentencing related to foreign collusion "appear to be on the high side".

"I think the Court is wrong to say that (pre-national security law) conduct can be considered in assessing the gravity of the offences," Young told AFP.

Lai's legal team should consider an appeal of sentence, he added.