Police in the southern city of Guangzhou arrested Yang Maodong, an advocate for legal reform, on a charge of "assembling a crowd to disrupt order in a public place", US-based group Chinese Human Rights Defenders said on a blog.
The group posted a photograph of a notification document for Yang's arrest, apparently issued on August 8. Yang, known by the pen-name Guo Feixiong, has campaigned for freedom of speech and aided villagers who accused local officials of corruption.
Yang's arrest comes less than an month after Beijing police detained human rights lawyer Xu Zhiyong, who had publicly called for the release of other activists arrested for demanding that government officials disclose their assets.
The latest arrest signals "the continued repression of human rights activists by Chinese authorities", Chinese Human Rights Defenders said.
Rights groups have reported a crackdown on political activists since President Xi Jinping was formally appointed in March.
At least 24 activists have been detained since late March, Chinese Human Rights Defenders said in a statement last month.
China's ruling Communist party has launched previous crackdowns on activists demanding political change, arresting dozens in 2011 after calls for anti-government street protests emerged online.
Police in the Tianhe district of Guangzhou, where Yang is said to have been detained, did not answer phone calls on Saturday.