Jimmy Lai, Hong Kong's pro-democracy media tycoon, has been jailed for 20 years for colluding with foreign forces under the city's controversial national security law.

Rights groups called it a death sentence for the 78-year-old, whose family has raised concerns about his health, but Hong Kong's leader said it was "deeply gratifying".

This is the harshest punishment to be handed down under the law, which China imposed after huge protests in 2019 demanding more freedom, and defends as essential for the city's stability.

Lai, a British citizen, is the most prominent of the hundreds arrested under the law. A fierce critic of China, he often wielded his paper, Apple Daily, as a tool of protest.

"It is incredibly heartbreaking," Lai's son, Sebastien, told the BBC's Today programme, adding that he had repeatedly raised Lai's imprisonment with the UK government but "obviously my father is still in prison".

Sebastien Lai and others have criticised Prime Minister Keir Starmer's recent visit to China as a wasted opportunity.

The sentence "signifies the total destruction of the Hong Kong legal system and the end of justice", Sebastien said.

Hong Kong authorities and China's foreign ministry, however, insist the Lai's sentencing demonstrates the city's rule of law. In sentencing Lai, the judges condemned his "serious and grave criminal conduct" and said he was involved in conspiracies that were of the "most serious" category.

On Monday morning, ahead of sentencing, there was a heavy police presence around the court.

But there were also throngs of Lai's supporters, some of them having camped out for days hoping to get inside for his sentencing.

"I just wish to greet Mr Lai in person in court... That's all I ask for, because I think it will be the last time I see him," said one supporter, who said he'd stood in line since Thursday night.

Inside, Lai, who wore a white jacket and black glasses, smiled brightly at his family and other supporters. He nodded calmly when his sentence was announced, while some members of the public were heard sobbing. His wife Teresa held back tears as she left the court.

Lai, who was convicted of fraud and unauthorised assemblies in an earlier case, has already been in jail for more than five years.

Six former Apple Daily executives and two activists were also sentenced on Monday under the national security law, with terms ranging from six years and three months to 10 years.

Lai has always denied the charges against him, saying he was advocating for what he believed were Hong Kong's values, such as rule of law and free speech.

The case hinges on a meeting Lai had with the US' then Vice-President Mike Pence and then secretary of state Mike Pompeo at the height of the 2019 protests to discuss the situation in Hong Kong.

But when he testified in November, Lai said he had "never" used his foreign contacts to influence foreign policy on Hong Kong, adding that he was "just relaying" the situation to them.

Rights groups have denounced the sentence as "draconian" and "egregious".

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

She called on the international community to "step up its pressure" to free Lai "if we want press freedom to be respected anywhere in the world".

Lai's case "shows the Chinese government's determination to crush independent journalism and silence anyone who dares to criticise the Communist Party", said Human Rights Watch's Asia director Elaine Pearson.

Caoilfhionn Gallagher KC, who leads Lai's international legal team but was not allowed to defend him in court, also urged the international community to pressure China

"Now that this sham trial is finally over, we call on leaders from around the world to speak with one voice in their demand for China to free Jimmy Lai so he can come home to his family in London at last," she told the BBC.

A number of countries - including the UK, Australia, the European Union and Japan - have already expressed concern, while United Nations human rights chief Volker Turk called for his release, given his age and health concerns.

China's foreign ministry spokesperson shrugged off criticism, saying "the relevant judicial cases are purely internal affairs" of Hong Kong.

"The central government firmly supports the HKSAR government in maintaining national security and punishing criminal acts which endanger it," the spokesperson added.

In his last interview as a free man in 2020, Lai told the BBC "I got everything I have because of [Hong Kong]".

"If this is payback time, this is my redemption," said Lai, who was then out on bail.

Born in the Chinese city of Guangzhou, Lai was 12 when he arrived in Hong Kong as a stowaway on a fishing boat. He started working menial jobs and eventually founded a multi-million dollar empire that included the clothing brand Giordano.

Lai began a new journey as a vocal democracy activist after China's crackdown on pro-democracy protesters in Beijing's Tiananmen Square in 1989.

He went on to launch pro-democracy news outlets like Apple Daily and Next magazine, while regularly participating in demonstrations.