Three Chinese anti-corruption activists were due to face trial Monday on charges of "illegal assembly", in a case seen as a sign of an official clampdown on citizens' demands for reforms.

Liu Ping, Wei Zhongping and Li Sihua were charged after unfurling a banner outside a housing complex in April, calling for government officials to disclose their assets, their lawyers said.

Such a reform is seen as a key change that could help prevent graft after reports of massive wealth amassed by officials and their families.

China's new leadership under President Xi Jinping has mounted a much-publicised campaign against corruption, with some high-ranking individuals prosecuted.

But no systemic changes have been implemented and the Communist Party, which maintains tight control over the political process, is highly intolerant of organised dissent.

Security was stepped up outside the courthouse in Xinyu, in the central province of Jiangxi, pictures posted online showed.

Defence lawyer Pang Kun told AFP that he had been detained by authorities for several hours on Sunday, alongside a writer who planned to witness the trial who was still unreleased.

Court officials and lawyers could not be reached for comment by AFP Monday.

Liu, 48, an unemployed former steel worker, was associated with the "new citizens movement", a loose grouping of activists calling for reforms to China's legal system, her daughter Liao Minyue said.

At least 15 others involved in the group have also been detained in recent months, US-based rights group Human Rights Watch said.

"Liu, Wei, and Li are canaries in the coal mine for how the government intends to treat this influential group of anti-corruption activists," Sophie Richardson, China director of the group, said in a statement.

In addition to the illegal assembly charge, Liu Ping faces charges of "gathering a crowd to disturb order in a public place", and "using an evil cult to undermine the law", her lawyer said.

All three defendants planned to plead not guilty to all charges, Liu's lawyer Zhang Xuezhong told AFP.

But the trial may be delayed as the defendants' lawyers may refuse to cooperate with the court in protest at the way the case had been handled, he said.