Authorities in Beijing have charged rights activist Wang Lihong with "picking a fight," as local rights groups called for her release.
Wang, 55, was formally detained by the municipal procuratorate following a month of informal detention, her son said.
"I went to the procuratorate a couple of days ago," said her son, surnamed Qi, confirming Wang's formal arrest. "The formal letter was supposed to arrive, but I haven't received it yet."
"I will have to go to my mother's place and see if it has arrived."
Wang was detained by police on March 21 in the wake of anonymous online calls for a "Jasmine" revolution, inspired by recent events in the Middle East.
Qi said he had hired top civil rights lawyer Liu Xiaoyuan to defend Wang, who is currently being held in Beijing's Chaoyang district detention center.
"He said we would go to the Chaoyang detention center next week," Qi said. "The last time we went, they wouldn't let us visit her."
Qi said he is concerned for his mother's health. "Her heart isn't very healthy and she has back problems," he said.
"They won't let me take her medications, because they have a clinic there."
Lawyer Liu was reluctant to comment on Wang's case.
"As you know, I have only just been released myself," he said. "There are some things which I can't really speak to you about frankly."
Active in many projects
According to the Chinese Human Rights Defenders (CHRD) website, Wang had been active in such projects as relief efforts for the “Tiananmen homeless” and advocacy in the case of Yang Jia, a Beijing man executed after he was accused of killing six Shanghai police officers.
She had also worked with legal scholar and activist Xu Zhiyong and others on the case of petitioner Yao Jing, who was hospitalized after being beaten by government officials.
An active Twitter user and netizen, she took part in protests outside the April 2010 trial of three Fujian netizens eventually convicted of “slander” and imprisoned for posting information online regarding the suspicious death of a young woman, CHRD said.
"In late 2010, she was administratively detained for eight days and then placed under more than three months of “soft detention” for celebrating Liu Xiaobo’s Nobel Peace Prize," the group said.
The group said China's ruling Communist Party has criminally detained 39 people since calls for a "Jasmine" revolution first appeared in mid-February.
Of that number, six have been formally arrested, three have been sent to labor camp, nine have been released on bail awaiting trial, and 17 remain detained, CHRD said.
Reported by Qiao Long for RFA's Mandarin service. Translated and written in English for Luisetta Mudie.