China's reformist New Citizens Movement is widening the attraction of activism with its low-key gatherings -- and its loose cell structure worries the mighty Communist state despite the group's tiny size, campaigners say.

Overt political advocacy in China is often associated with a small circle of die-hards, but the amorphous movement instead aims to make the everyman an activist, prompting a crackdown by protest-wary authorities.

Its first three self-described members to go on trial were in court this week on charges of "illegal assembly" after they displayed a banner calling for officials to disclose their assets as a check against corruption.

China's President Xi Jinping has launched a much-publicised campaign against graft, but the ruling Communist Party stamps out any activity that might snowball into protests challenging its rule, and has begun picking off New Citizens.

Nonetheless the network is so diffuse that neither members nor observers can say how many people have been detained, or even how many are involved.

"The New Citizens Movement does not have strict organisation or a constitution, nor does it have clear leaders or set members," said Teng Biao, a well-known rights lawyer and central figure in the group.

"It's an Internet-based movement with lots of hubs," he said. "Any citizen who wants to take on the responsibilities of a citizen can participate."

The movement was articulated in mid-2012 by prominent Beijing rights lawyer Xu Zhiyong, who was detained in July this year.

He described a political, social and cultural grouping that "destroys the privilege of corruption, the abuse of power... and builds a new order of fairness and justice".

Citizens were neither "subjects" nor "mobs", he said, but defenders of a social contract who met over dinner to discuss issues and, for riskier types, took to the street to express their demands.

He urged Chinese to "spread the New Citizen spirit" through the Internet, street fliers and T-shirts, discuss policy issues over a meal, and uphold rights by filing lawsuits and photographing injustices.

Such activities were already happening but Xu extended their appeal by providing a vision and branding.

He created a logo -- the characters for "citizen" as written by Sun Yat-sen, considered the founder of modern China and also a hero to the Communist party.

Teng estimates that monthly dinners have taken place in 30 cities with 20 to 200 people attending each, and that more than 40 people have been detained, while US-based campaign group Human Rights Watch counts at least 16.

Participants range from lawyers to labourers, Teng said, as well as longtime dissidents who have served time in prison.

Among the most involved "laypeople" was Wang Gongquan, a billionaire businessman and friend of Xu detained in September.

High-profile Beijing dissident Hu Jia, who has joined in events, said that by being open and accessible the movement could have long-term sustainability.

But it has been accused of being both too moderate for seeking change within the system and too confrontational, causing a spike in arrests, he added.

At the dinners participants propose their own topics for discussion, from government land seizures to labour camps, and follow Robert's Rules of Order -- a 19th century US guide to meetings procedure -- as a form of "democracy training".

Prominent causes have included urging authorities to publicise their assets and improving migrants' access to education in cities.

China's new leadership under Xi has embraced similar issues, but since it took office ordinary Chinese who have done so in an organised way have been taken in on charges of public disorder or illegal assembly.

"They worry that the New Citizens Movement will spread, and that through the dinners lots of people will discuss political problems, and form an organisation, and the organisation will evolve into a street movement -- and street movements are what they fear most," said Hu.

"So any time these kinds of people appear they inevitably crack down."

Human Rights Watch researcher Maya Wang said the crackdown had dealt "quite a blow" and activities had fallen off.

But one advantage of the New Citizens model as an "organisation-less organisation" was that it could survive individual arrests, said Eva Pils, an associate professor with the Centre for Rights and Justice at the Chinese University of Hong Kong.

"Having no leadership means that sending one person to prison or detaining them temporarily doesn't mean that everything breaks down," she said.

"It has this ability to almost disappear and reappear about as fast as it takes people to just connect online."