Police Friday detained an outspoken Chinese businessman who had called for the release of a prominent lawyer arrested as part of a crackdown on anti-corruption activists, a friend told AFP.

Wang Gongquan was detained at his home in Beijing on suspicion of "assembling a crowd to disrupt order in a public place", writer Xiao Shu said, adding a Wang family member told him of the arrest.

Wang, a wealthy venture capitalist, had publicly called for Xu Zhiyong, a lawyer who had assisted other campaigners arrested for demanding that government officials disclose their assets, to be freed.

Rights groups have reported a crackdown on political activists since President Xi Jinping was formally appointed in March, with at least 30 said to have been detained.

Some of them have been charged with "assembling a crowd to disrupt order in a public place" after they appeared on city streets with banners calling for officials to disclose their wealth.

Corruption is a key issue in China that Xi has said threatens the ruling Communist Party.

Wang has long been an outspoken advocate for political reform, and activists told AFP that he had provided financial backing for Xu. He also started a signature campaign for the release of the lawyer, who was detained in July and has not been seen in public since.

Wang's own arrest was confirmed to AFP by fellow activists Hu Jia and lawyer Teng Biao, who said around 20 police were involved.

Reportedly a billionaire, Wang made waves in 2011 when he announced he was retiring to enjoy life with his mistress, and news of his arrest spread rapidly on Chinese social networking websites Friday.

Political commentator Peng Xiaoyun wrote on Sina Weibo, a social media service similar to Twitter: "Do not think that silence is harmony.

"The greater the pressure limiting people, the more those who are restricted will gain moral and political energy, while the public will lose confidence in the rule of law."

Beijing police were not immediately available to comment to AFP.