Yang, a leader of the China Human Rights Defenders, was picked up Aug. 8 for allegedly disrupting public order by assembling a crowd in a public place.
Lawyer Sui Muqing told The New York Times by telephone the government did not notify Yang's family about the arrest until Saturday.
Sui told the newspaper the detention was likely the result of public protests held in January in support of the Southern Weekend, a Guangzhou newspaper locked in a dispute over government censorship.
"I think the general reason is that the authorities are putting pressure on many dissidents, and Yang Maodong is one of them," Sui said. "I think the direct reason may be his involvement in the protests at the Southern Weekend."
The Times said Yang, a former novelist known by his pen name, Guo Feixiong, was the second well-known member of China's pro-rights movement to be detained by authorities this summer. Legal advocate Xu Zhiyong was taken into custody in July.