China has detained a lawyer known for his anti-corruption and human rights campaigns, reports say.
Xu Zhiyong was detained on Tuesday on suspicion of having "gathered crowds to disrupt public order", rights groups and media report.
The police also confiscated computers and phones from his home in Beijing, US-based Human Rights in China says.
Mr Xu, who has campaigned for government transparency, was placed under house arrest on 12 April.
He said in an email to Reuters news agency in May that the house arrest "could be due to my campaign to push for asset disclosure" of government officials.
Wang Weiguo, Mr Xu's lawyer, questioned the allegations against Mr Xu.
"How can a man who has been held under house arrest disrupt public order?" he told the South China Morning Post newspaper.
He added that Mr Xu had "never advocated violence in his writings" and "just availed himself of his right of freedom of speech as an ordinary citizen".
Xu Zhiyong is seen as a leading advocate of a group campaigning for government officials to reveal their wealth.
Several members of the group have been arrested in recent months.
Human rights groups have described the move as the first major crackdown by the country's new leaders, says the BBC's Martin Patience in Beijing.
It has been seen as a signal that public dissent will not be tolerated under new President Xi Jinping, our correspondent adds.
Since replacing Hu Jintao as China's president in May, Mr Xi has initiated a high-profile campaign against corruption among officials.
"This crackdown not only flies in the face of Xi's rhetoric, it also undermines Xi's legitimacy," Human Rights Watch researcher Maya Wang told Reuters news agency.
"The right thing to do would be for the government to release the activists."
Mr Xu, a known legal scholar, has campaigned in behalf of inmates on death row and families affected by tainted baby milk formula in 2009, among other causes.
In 2009, he was arrested on tax evasion charges which were eventually dropped after public outcry.