Lai, who turns 78 next Monday, has been behind bars since late 2020 as China clamps down on the financial hub to which it promised a separate system when Britain handed it over in 1997.

Lai, a diabetic, has been kept in solitary confinement without air conditioning in a jail where summer temperatures rise to 44 Celsius (111 Fahrenheit), his children said.

"He has lost a very significant amount of weight, visibly, and he is a lot weaker than he was before," said his daughter Claire Lai, who left Hong Kong after seeing her father several months ago.

"His nails turn almost purple, gray and greenish before they fall off, and his teeth are getting rotten," she told AFP on a visit to Washington, where the family is seeking to rally support for her father.

Hong Kong's government condemned what it called "fact-twisting" reports on Lai's treatment, including from AFP.

"These reports seek to smear (Hong Kong's government) by misleading the public," the Chinese territory's authorities said in a statement.

"The HKSAR government has emphasised time and again that the medical services received by Lai... in custody are adequate and comprehensive," they added.

Claire Lai said that guards refused to let her father, a devout Catholic, receive communion and that they made small gestures to try to demoralize him.

After learning he enjoyed curry sauce, "instead of having extra curry sauce, he has no curry sauce at all," she said.

"It's little things like that that are extremely petty," she said.

A successful businessman, Lai founded the Apple Daily, an outspoken pro-democracy tabloid.

He faces at least 15 years in prison -- effectively a death sentence -- on charges of foreign collusion related to mass protests in Hong Kong in 2019 against Beijing's encroaching power.

His son Sebastien Lai voiced hope that both US President Donald Trump and British Prime Minister Keir Starmer would keep raising with China the issue of his father, who is a British national.

"It will take two hours to put my father on a plane and send him away," Sebastien Lai said.

"It'll be the humane thing to do; it'll be the right thing to do," he said. "They've already put him through this hell."