Jimmy Lai, a once mighty media mogul and prominent pro-democracy activist, has been sentenced to 20 years in prison in Hong Kong for national security offences.

The sentencing is the culmination of a years-long saga that critics say represents Hong Kong’s transformation from a mostly free city to one where dissent is fiercely suppressed by the Chinese Communist party-controlled authorities.

Lai, 78, was convicted in December on charges of sedition and conspiracy to collude with foreign forces. He had pleaded not guilty to all charges. The collusion convictions carry a maximum penalty of life imprisonment.

Lai’s sentence was immediately condemned by press freedom and human rights groups. A statement from Reporters Without Borders said: “Today, the curtain falls on press freedom in Hong Kong… This court decision underscores the complete collapse of press freedom in Hong Kong and the authorities’ profound contempt for independent journalism.”

Human Rights Watch also put out a statement calling the length of jail time given to Lai “effectively a death sentence”.

“A sentence of this magnitude is both cruel and profoundly unjust. Lai’s years of persecution show the Chinese government’s determination to crush independent journalism and silence anyone who dares to criticise the Communist Party,” the statement said.

Lai’s prosecution has been described as politically motivated by human rights groups and the British government, which has called for his release.

On Friday, Chinese state media described Lai, a British citizen who has lived in Hong Kong since he fled China as a child refugee, as an “anti-government instigator and traitor”.

Lai is the founder of the now defunct Apple Daily, a popular Hong Kong newspaper that supported the pro-democracy movement that swelled in the city in the 2010s.

The movement was crushed in June 2020 by the imposition of a harsh national security law which criminalised most forms of dissent. Lai was arrested and charged under that law in August that year. Apple Daily was forced to close in 2021.

The UK prime minister Keir Starmer said that he raised Lai’s case when he met China’s leader, Xi Jinping, in Beijing in January. But it is not clear what, if any, progress the UK has made towards securing Lai’s release. Lai’s son, Sebastien Lai, said recently that the UK government had not done enough for his father and that “time is running out”.

The US president, Donald Trump, has also said that he would secure Lai’s release.

There are increasing concerns about Lai’s health and wellbeing in prison. His family said that he has suffered dramatic weight loss and that he teeth have rotted. He has been behind bars since December 2020 and has spent most of that time in solitary confinement. Hong Kong authorities say Lai has received appropriate medical care and that he requested to be kept separate from other prisoners.

Lai was sentenced along with eight other co-defendants – two activists and six former executives from Lai’s media company – in the landmark national security trial, all of whom pleaded guilty.

Lai’s trial was presided over by three judges handpicked by the government to hear national security cases. In their 855-page judgment convicting him, the judges said that Lai was “a very savvy business man” and that it was “unfortunate that his deep resentment and hatred for the Chinese Communist party … led him down a thorny path”.

The judges said that Lai was someone who was “keen on tearing down the CCP”.

Lai was accused of using Apple Daily and political connections, particularly in the US, to lobby for foreign governments to impose sanctions on China and Hong Kong after the crackdown on pro-democracy protests in 2019 and 2020.

Lai said he never called for sanctions after the national security law had taken effect, as “it would be suicidal to do so”.