HONG KONG — The police in southern China have detained an outspoken advocate of democratic rights on charges of disrupting public order, his lawyer and human rights groups said Saturday. They called his detention another step in a government effort to stifle dissidents who have challenged the Communist Party over censorship and officials’ wealth.

The advocate, Yang Maodong, who is better known by his pen name, Guo Feixiong, was detained on Aug. 8, but the police notified his family only on Saturday, his lawyer, Sui Muqing, said by telephone from the city of Guangzhou, where Mr. Yang also lives. Chinese Human Rights Defenders, an international advocacy group, also reported Mr. Yang’s detention on its Web site, and it posted a copy of the police notice listing charges of “assembling a crowd to disrupt order in a public place.”

Mr. Yang, 47, is the second well-known member of China’s “rights defense” movement to be arrested recently. In mid-July, the police in Beijing arrested Xu Zhiyong, a legal advocate who has also long been a prominent member of that loose campaign, which seeks to expand citizens’ rights through litigation, petitions, publicity and training.

“I think the general reason is that the authorities are putting pressure on many dissidents, and Yang Maodong is one of them,” Mr. Sui said. “I think the direct reason may be his involvement in the protests at the Southern Weekend.”