The 76-year-old media tycoon has been testifying for three days in Hong Kong about claims that he colluded with foreign forces, an offence carrying up to life in prison under the sweeping national security law Beijing imposed on the financial hub in mid-2020.

The founder of the now-shuttered tabloid Apple Daily is also accused of "conspiracy to publish seditious publications".

His son Sebastien, who has been travelling the world to denounce his father's arrest since he was detained in 2020, is following in real time the media coverage of his father's testimony.

"The trial is one of the first times in the last four years where I have any indication of how he's doing," Sebastien Lai told AFP in an interview in London.

"He's been kept in solitary confinement for the last four years. His health obviously has gotten quite a bit worse but from his statements, his spirit's still strong and his mind's still sharp."

On Tuesday, Jimmy Lai told the court he had advocated "peaceful resistance" against China's erosion of the city's freedoms and tried to "reduce violence" during huge democracy protests in 2019.

"It's quite heartbreaking seeing my father being dragged to court with all these sham allegations. But I'm also incredibly proud of him"," Sebastien Lai said.

"They've been trying to break him for four years but he's unbroken."

- Hong Kong 'on trial' -

Sebastien Lai, who has not set foot in Hong Kong for four years because of his fight to defend his father, said he had little hope of seeing him judged impartially.

"It's a complete show trial," he said, noting that the three judges hearing the case were appointed by the government and there is no jury.

United Nations human rights experts have called for the release of Lai, as have major Western governments including Britain.

British Prime Minister Keir Starmer raised Lai's case with Chinese President Xi Jinping when they met on the sidelines of the G20 summit in Brazil this month.

"My father at every turn, at every crossroad, chose to do the right thing. And Hong Kong is showing you what the cost is of doing the right thing," Lai said.

Last week, a Hong Kong court sentenced 45 pro-democracy activists to up to 10 years in prison for subversion.

Lai says his father's trial demonstrates "a very clear decision by the government to criminalise everything that my father stands for, which is campaigning for democracy, the free press, free speech, freedom of assembly".

"Symbolically, they're putting my father on trial, but also these freedoms that made Hong Kong great on trial," he said.

Lai said statements by Starmer and foreign minister David Lammy calling for his father's release were "incredibly important" but did not go far enough.

"The United Kingdom can decide whether they are a place that protects citizens from being arbitrarily detained or not," he said, adding that the fact his father will turn 77 next month makes his case "very urgent".

"My father could die at any point given his age. It's not just about freeing him now. It's about saving his life."