US President Donald Trump says he has asked Chinese leader Xi Jinping to "consider" releasing Jimmy Lai, the Hong Kong pro-democracy tycoon who was found guilty under the city's controversial national security law earlier this week.

"I feel so bad," Trump told reporters. "I spoke to President Xi about it and I asked to consider his release."

The UK similarly called for the 78-year-old to be "immediately released, with UK Foreign Secretary Yvette Cooper condemning the ruling, calling it a "politically motivated persecution.

Lai, who is a British citizen, has been in jail since December 2020 and is due to be sentenced early next year. He faces a maximum term of life in prison.

Trump made the brief comment to reporters at the White House on Monday, but did not specify when he had brought up Lai's case with Xi.

"He's an older man, and he's not well. So I did put that request out. We'll see what happens, okay?" he said.

His comments come after a Hong Kong court on Monday found Lai guilty of colluding with foreign forces.

The verdict was welcomed by Hong Kong's chief executive John Lee, who said that Lai's actions had "damaged the country's interests and the welfare of Hong Kongers", but rights groups called it "a cruel judicial farce".

"He has been targeted by the Chinese and Hong Kong governments for peacefully exercising his right to freedom of expression," UK Foreign Secretary Yvette Cooper had told Parliament on Monday, calling for Lai's "immediate release".

She added that the Foreign Office has summoned the Chinese Ambassador to "underline our position in the strongest terms", adding that it was "heartbreaking that such a violation of a British man's rights could occur in Hong Kong".

China's foreign ministry had earlier on Monday dismissed criticisms of Lai's trial, describing them as "brazen defamation and smearing of the judicial system in Hong Kong".

Lai was convicted on Monday under the city's controversial national security law, which rights groups say is used to crush dissent, but which Beijing defends as essential for the city's stability.

He was also convicted of publishing seditious material on Apple Daily under a separate colonial-era law.

His detention and conviction have drawn concern from rights groups and foreign leaders, who have long called for his release.

His children have voiced serious concerns about Lai's deteriorating health in prison, with Sebastien telling the BBC earlier this year that given his father's age and health, he could "die in prison".