Yang Lin, 45, a critic of China's one-party system who lives in the southern province of Guangdong, was arrested on a charge of "inciting subversion of state power", his brother, Yang Mingzhu, said by telephone.
In China, an inciting subversion charge is commonly levelled against critics of one-party rule. It carries a maximum penalty of five years in jail, though lengthier sentences have been handed down.
Yang Mingzhu said he had received a notice of his brother's arrest, dated July 19, but it gave few details.
The US-based group Chinese Human Rights Defenders said Yang Lin, had spent a year in a labour camp, and he was also a signatory of "Charter 08" - a manifesto organised by jailed Nobel Peace Prize Laureate Liu Xiaobo - which calls for political reform.
"He would not hesitate in throwing himself wholeheartedly in helping disadvantaged citizens fight for their rights and in activities promoting constitutional democracy," the advocacy group said on its website on Sunday.
Chinese liberals and intellectuals had hoped the new government that took over this year, under President Xi Jinping, would be more tolerant of calls for reform but authorities have seemed to indicate they will not tolerate any challenge to their rule.
In recent months, authorities have detained at least 16 anti-corruption activists involved in demonstrations calling for government officials to disclose their assets.
The Futian District Detention Centre, where the brother said Yang Lin was being held, declined to comment.
A formal arrest usually leads to a trial. Activists who are detained are sometimes released before they are formally arrested.
In June, authorities formally arrested a man for inciting subversion after he applied for permission to demonstrate on June 4, the 24th anniversary of the bloody crackdown on protesters in Beijing's Tiananmen Square.