BEIJING (REUTERS) - Three Chinese anti-graft activists who agitated for officials to disclose their assets go on trial on Monday, in the first case of its kind that underscores the limits of the government's tolerance of challenges to its authority.

Despite an official drive against corruption, China has detained at least 15 activists in recent months who were involved in a campaign pushing for officials to publicly disclose their wealth.

Rights groups describe the detentions as the first major crackdown by the new government on activists.

Liu Ping, Li Sihua and Wei Zhongping were detained in late April in Xinyu, in the poor landlocked southern province of Jiangxi, and accused of illegal assembly. They face a maximum of five years in prison if convicted.

Officials in Xinyu could not be reached for comment.

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

"Anything short of acquittal will seriously undermine the credibility of the government's claims to be cracking down on corruption."

The ascendancy of Xi Jinping as Communist Party chief in a once-in-a-decade generational leadership transition last November had given many Chinese hope for political reform, spurring citizens to push officials to disclose their wealth in several movements throughout the country.

But the charges against the activists are a strong indication that the Communist Party will not tolerate any open challenge to its rule under Xi, even as it claims more transparency.

Xi, who became president in March, has called for a crackdown on graft, warning, as many have before him, that the problem is so severe it could threaten the party's survival.

Encouraged by Xi's calls for more transparency, the activists took photographs of themselves holding banners and placards that said: "Strongly urge officials to disclose their assets" and "Xi Jinping, immediately end dictatorship".

The pictures were widely circulated online.

There have been a few pilot schemes for low-level officials in China's southern Guangdong province to disclose their assets, but the efforts have made little progress and discussion of the wealth of senior leaders such as Xi remain firmly off limits.

Zhang Xuezhong, one of two lawyers defending Liu, said the authorities had thumbed their noses at legal procedure ahead of the trial, which boded badly for the case.

There was evidence of coordination and cooperation between the police, the investigators and the court, and police and state security visited his law firm and talked with the firm's boss on multiple occasions and pressured him to drop this case, Zhang told Reuters.

"We discovered serious procedural problems," he said. "I am not optimistic." Zhang added he expects they will all be found guilty.

Authorities first accused Liu of "gathering to disturb social order", then "inciting subversion of state power", and later "illegal assembly" - proof the government's move against the activists was out of retaliation, his other lawyer, Zheng Jianwei, previously said.

Li Sihua's lawyer, Pang Kun, told Reuters on Sunday that he had been detained at the police station, though he had not been given a reason why.

"I should still be allowed in court as they have no right to prevent me going," Pang said, before being interrupted by what sounded like a scuffle in the background and shouts of "who are you?" and "what are you doing?" before the line went dead.

Efforts to contact the other lawyers by their mobile phones were unsuccessful.