Police in Beijing have detained Hu Jia, an outspoken government critic and friend of the Chinese activist Chen Guangcheng who escaped house arrest last weekend, Mr Hu's wife said on Sunday on Twitter.
Overnight on Sunday, Zeng Jinyan said "people from the police will come to collect anti-viral medicine. His (Hu's) detention has been extended to 24 hours. I asked where Hu Jia would sleep, they said on a chair."
Later on Sunday, Ms Zeng said state security in Tongzhou District, east Beijing, had called her again, "demanding to see me today to speak to me.
"I have had enough of these never-ending conversations, accompanied by meals and alcohol. That's enough!" she said on Twitter, adding that she did not know what to do with their child.
Hu Jia, well-known for activism in support of human rights, people living with AIDS and the environment, was released in June 2011 after more than three months in prison for "attempted subversion of power."
Mr Chen, 40, has won worldwide acclaim for exposing forced sterilisations and late-term abortions under Chinese policy, and for using his legal knowledge to help commoners battle a range of other perceived injustices.
The self-taught lawyer escaped house arrest last Sunday in the eastern province of Shandong with the help of his supporters, and subsequently recorded a video alleging abuses against him and his family.
Mr Chen may have fled into the US embassy in Beijing.
China Aid, a group run by former Tiananmen Square democracy activist Bob Fu, said it had learned from a "source close to the Chen Guangcheng situation" that the activist was now "under US protection".
The United States has expressed concern about Mr Chen, but refused any comment on his whereabouts, underscoring the huge sensitivity of the issue.
A decision to grant him refuge could prove a major diplomatic irritant, with US Secretary of State Hillary Clinton and Treasury Secretary Timothy Geithner due in Beijing on Thursday for annual talks on their often testy relationship.
The last Chinese dissident known to have been granted refuge at the US embassy was Fang Lizhi, a key figure in the pro-democracy movement who spent a year under US protection after publicly supporting the 1989 Tiananmen protests.
He was forced into exile in 1990 and died in the United States earlier this month.