A professor who criticised China's handling of the coronavirus crisis has been detained by authorities.
Xu Zhangrun, who has been under house arrest, was taken away from his Beijing home on Monday, friends said.
The law professor has previously spoken out against the Mao-like cult of personality which has returned under China's current leader, Xi Jinping.
Police have not commented publicly on the arrest, and it is unclear what charges Mr Xu faces.
One friend told news agency AFP that Mr Xu's wife had received a call telling her he was accused of soliciting prostitution while in the city of Chengdu with other liberal academics.
The friend dismissed the allegation as "ridiculous".
Freedom of expression is tightly controlled in China, with those who speak out against authorities risking arrest and jail sentences.
Friends say as many as 20 people appeared at Mr Xu's house early in the morning, seizing his computer and papers.
Geng Xiaonan, a friend of the professor, told the New York Times he was "mentally prepared to be taken away".
"He kept a bag with clothes and a toothbrush hanging on his front door so he would be ready for this," she said.
A BBC Beijing correspondent says Mr Xu has been treading an ever more dangerous path.
He had been barred from teaching at Tsinghua University - one of the country's top institutions - after he spoke out against the removal of presidential term limits, allowing Mr Xi to remain in post for life.
He was placed under house arrest earlier this year after publishing an article criticising the way President Xi and the government had handled the coronavirus outbreak. He suggested it might be the last one he ever wrote.