The Chinese lawyer Xu Zhiyong has been detained after calling for the release of activists and campaigning against government abuses, according to a rights group and a fellow lawyer.
Police also took computers and mobile phones from Xu's Beijing home, Human Rights in China said.
A scanned detention notice distributed by the New-York-based group said Xu was detained on Tuesday on suspicion of gathering people to disturb order in a public place.
No details or causes were given to support the vaguely defined charge, which is often used to punish people who speak out against abuses.
Teng Biao, another prominent Beijing human rights lawyer, confirmed Xu's detention for disturbing public order on Twitter. Teng said his telephone had been frozen and he could only communicate by computer.
Beijing police did not immediately answer faxed questions about Xu, who has campaigned for causes ranging from the closure of illegal detention centres to the public declaration of officials' personal assets.
Officers at Beijing's No 3 jail, where Xu was reportedly being held, said they were not permitted to speak to the media.
Xu has frequently been punished for his outspokenness with lengthy periods of house arrest, the most recent beginning on 12 April after he signed a letter in support of other detained activists.
China's Communist party rulers tolerate no challenge to their rule and frequently punish critics with lengthy prison sentences, such as the 11-year term given to the Nobel peace prize winner Liu Xiaobo for co-authoring a call for sweeping political reforms.
Authorities say they protect citizens' legal rights and point to vast improvements in incomes and quality of life achieved over the past three decades.
