China is relentlessly cracking down on activists despite criticism from the international community over its human rights record.
Rights lawyer Ni Yulan and her husband were taken away by police from a hotel in a western suburb of Beijing on Thursday, joining a long list of activists detained in an ongoing clampdown on groups such as bloggers, human rights lawyers, and writers.
"This morning they ... took away Ni Yulan and her husband," said Beijing-based activist Liu Shasha. "It happened first thing this morning."
Ni's daughter Beibei said her parents had been incommunicado all day.
"I tried to call them, and then I went over there," she said. "There were 30 or 40 police there."
She said she was taken to the police station for two hours, where her mobile phone was apparently disabled, but she wasn't interrogated.
"I don't know where they've been taken ... No one has been given any information."
"I can't switch on my phone now," she said. "They told me not to give any interviews for the time being."
Ni has been homeless alongside her husband Dong Jiqin since being released from a two-year jail sentence in April 2010.
The authorities jailed her and demolished the couple's home after she represented a group of housing activists complaining about forced evictions.
U.S. Ambassador weighs in
Outgoing U.S. ambassador to China Jon Huntsman had visited the couple at the Yuxingong Guesthouse in February after authorities pressured the hotel to switch off their electricity.
One Beijing rights activist who asked to remain anonymous said Ni's detention was likely linked to Huntsman's Shanghai speech on Wednesday.
"Huntsman had taken notice of Ni's case," said the activist. "Now he's made a strong protest against human rights abuses."
"I think they're saying that if he's going to do that, we'll take it out on her," the activist said.
In an annual lecture in Shanghai, Huntsman hit out at the detention and jailing of prominent dissidents, including Nobel peace laureate Liu Xiaobo and detained artist and social critic Ai Weiwei.
Ai, held at the weekend, was the most high-profile detention since Beijing launched a crackdown on government critics after anonymous calls for a "Jasmine Revolution" in China, partly inspired by pro-democracy movements in the Middle East, began circulating online in mid-February.
"The United States will never stop supporting human rights because we believe in the fundamental struggle for human dignity and justice wherever it may occur," said Huntsman, amid speculation he may seek the Republican nomination for president on leaving his post this month.
"Cutting off dialogue and suppressing the news media do not help," Huntsman said, in what is rare public criticism for a diplomat of his rank.
Many governments, including in Europe, have joined the United States in criticizing Beijing over the crackdown.
Activist tells of detention
Meanwhile, tainted milk activist Zhao Lianhai said via the microblogging service Twitter that he was detained on Wednesday after breaking his silence for the first time since his release from jail on "medical parole."
Zhao recorded an emotional 20-minute video message which he posted online, calling for Ai's release.
"My feelings have been very painful recently, seeing lots of my good friends being strictly treated and suppressed," Zhao said in the video.
His wife Li Xuemei later reported via his Twitter account that he had been released.
Beijing-based lawyer Li Fangping said Zhao came under a "great deal of pressure" from the Chinese authorities after he gave interviews to foreign media in which he called for the release of detained prominent artist Ai Weiwei.
"He is bound to have run into some trouble as a result of these interviews," Li said.
Liu Mengxiong, a delegate to the Chinese People's Political Consultative Conference (CPPCC), who has followed Zhao's case closely since the 2008 milk scandal first came to light, said Ai had been instrumental in supporting Zhao when he was arrested, and that Zhao had felt unable to remain silent when Ai himself was detained.
Liu said the exact terms of the deal under which Zhao was released on medical parole haven't been made public.
"At the very least I think they will send people to talk to him and step up surveillance on him," Liu said.
"They probably won't just throw him back in jail, because his case has attracted a lot of concern overseas."
"If they took stronger measures against him, I think they would cause an outcry."
Hong Kong lawyer and Democratic Party legislator Albert Ho said Zhao had probably breached the terms of his medical parole, which were to have no contact with foreign media.
"I just hope that they won't take revenge against him," Ho said. "The things he said were pretty moderate."
Reported by Qiao Long for RFA's Mandarin service and by Wen Yuqing for the Cantonese service. Translated and written in English by Luisetta Mudie.