Three Chinese anti-corruption activists who unfurled banners calling for government officials to declare their assets stood trial Tuesday for "illegal assembly", a lawyer said, despite an official campaign against graft.

An anti-corruption drive championed by China's President Xi Jinping has been heavily publicised in state-run media, but the ruling Communist party keeps a tight grip on political dissent and the case is seen as part of a clampdown on citizens who publicly demand reforms.

Liu Ping, Wei Zhongping and Li Sihua -- charged after taking photographs of themselves displaying the banners outside a housing complex in April -- all pleaded not guilty, one of their lawyers told AFP, adding that illegal assembly carries a maximum sentence of five years in prison.

Asset disclosure is seen by some as a key change that could help China prevent graft after reports of enormous wealth amassed by officials and their families.

Si Weijiang, an attorney for Liu, said that at the hearing "we discussed the facts of the charge of illegal assembly, which are somewhat ridiculous -- they just took photos outside an apartment.

"It has become a secret hearing, with only two family members let into the court to observe proceedings," he said. "The court has violated regulations on holding open trials."

The court in Xinyu, in the central province of Jiangxi, could not be reached for comment by AFP Tuesday.

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

Liu was arrested in May, she said, adding that police had been stationed at her house ahead of the trial and followed her when she ventured outside, with one officer threatening to beat her.

"If the law was really respected, this case would never have gone to trial, so I don't have any hope for a just verdict," she said.

At least 15 others involved in the group have also been detained in recent months, according to US-based rights group Human Rights Watch (HRW).

The case is "a bellwether of the new Chinese leadership's attitude toward peaceful activism", HRW said in a statement, adding: "Anything short of acquittal will seriously undermine the credibility of the government's claims to be cracking down on corruption."

The trio's trial was originally due to take place last month, but was suspended after they withdrew permission for their lawyers to represent them in protest at what they called the court's "illegal conduct", including banning some witnesses from appearing.

One of Li's replacement attorneys Pu Zhiqiang accused the court of again breaking regulations by not letting witnesses who were allowed at the earlier hearing testify in the new proceedings.

"Everyone knows these people did not commit any crime," he said. "We think this is a chance for the courts to improve their image."

Liu Ping faces further charges of "gathering a crowd to disturb order in a public place", and "using an evil cult to undermine the law", Si said, adding that the hearing was likely to continue Wednesday.