Ho, 71, formerly led the now-disbanded Hong Kong Alliance, which organised an annual candlelight vigil for more than three decades to mourn the victims of China's bloody crackdown at Tiananmen Square in 1989.

He was arrested by Hong Kong's national security police over alleged witness tampering, a police source told AFP.

The lawyer is already facing up to a decade in jail over an "incitement to subversion" charge under the national security law, which Beijing imposed on Hong Kong in 2020 after widespread and sometimes violent pro-democracy protests.

"We are following ongoing cases under national security law with great concern," the UN Human Rights Office said in a tweet.

"A week ago, Albert Ho was returned to custody under this law, despite his critical health condition. We urge authorities to release Ho, to continue his urgent medical care."

The trial in Ho's subversion case is pending, and he was granted bail in August after spending almost a year in prison. His bail conditions included a gag order on any speech deemed a threat to national security.

Ho's arrest was the latest in a series of high-profile detentions under the national security law.