The London-based rights group Amnesty International says feminist and labor activist Li Qiaochu, who has been incommunicado since February, is at risk of torture or inhumane treatment.
"No contact has been made with [Li] since she was taken away by Beijing police," the group said in a call for urgent action on its website.
It said Li is being held under "residential surveillance at a designated location [RSDL]," in an unknown location, and has been denied contact with family, friends, or a lawyer.
"There are grave concerns that Li Qiaochu is at risk of torture or other ill-treatment as she is still without access to her family or a lawyer of her choice," the group said.
It called on its members to write to the Beijing municipal police department calling for Li's immediate and unconditional release.
Amnesty International's warning came after Li's lawyer went to her local Dongxiaokou police station on April 21 to try to find out more about her case.
"Her lawyer still does not know anything about her whereabouts, wellbeing, or even the charges against her," Amnesty said, adding that there is no way of knowing if Li has prompt, regular, and unrestricted access to medical care.
"As she has suffered from depression since June 2019, [we] worry about her mental and physical wellbeing if she is not getting the appropriate care," Amnesty said in a suggested letter for campaigners to use.
Illegal behavior by police
Beijing-based rights activist Hu Jia said Li's case has been marked by illegal behavior by police since the beginning.
"It is actually the state security police in charge of the case, but they are hiding in the background and won't show their faces," Hu said. "They don't want the lawyer to meet with their client."
"The police at Dongxiaokou police station are blatantly preventing any information from getting to the lawyer," he said. "We don't know where she is being held."
"The police just issued a notice of RSDL and had the family sign it."
Li's detention came after she commented via her Twitter account on Jan. 24 on the attempted cover-up by authorities in the central province of Hubei of the emerging coronavirus epidemic.
"Please can we remember those who suffered, died, and those who died before they even had time to become a statistic," she wrote.
"We shouldn't be singing anyone's praises during this time of disaster; rather we should collectively participate in chasing those who were so careless with people's lives."
Helped workers, women
Li worked to find accommodation for thousands of migrant workers forcibly evicted from their homes by authorities in Beijing during a bitterly cold winter in 2017.
She also played an active role in China's #MeToo movement, collating and publishing reports of sexual harassment and abuse online.
After the epidemic struck, Li joined a volunteer team that handed out free masks to sanitation workers in Beijing, and helped pregnant women in quarantine areas to find doctors.
She also worked to support victims of domestic violence, which saw a nationwide spike after hundreds of millions of people were placed under draconian quarantine lockdowns in central China.
Li was summoned by police and held for questioning for 24 hours on Dec. 31 as part of a nationwide operation targeting a group of activists who met in the southeastern port city of Xiamen on Dec. 13.
As well as being a prominent feminist and labor activist, Li is the partner of detained New Citizens' Movement founder Xu Zhiyong, who is also being held by state security police on suspicion of "incitement to subvert state power" after he called on President Xi Jinping to resign.
Xu, who has already served jail time for his spearheading of the New Citizens' Movement anti-corruption campaign, penned an open letter to Xi while in hiding following an earlier meeting in Xiamen, calling on him to step down.
A source close to Xu said he is also being held incommunicado under RSDL, a status that allows police to hold anyone they say is suspected of crimes linked to national security without contact with family or a lawyer for up to six months.
Reported by RFA's Mandarin Service. Translated and edited by Luisetta Mudie.