A group of museums commemorating China's violent Cultural Revolution is opening up normally tightly controlled discussion of the chaotic era -- but only up to a point.

Businessman Fan Jianchuan has opened six museums about the ten year period beginning in 1966 when China's then-leader Mao Zedong called on ordinary citizens to struggle against entrenched interest groups -- including government officials.

The 55-year-old says he's filled six warehouses with artefacts from the period, when young people formed often violent "Red Guard" groups and those labelled as "capitalist roaders" were publicly tortured at mass rallies.

"I see myself as an archaeologist of the Cultural Revolution," Fan, a former government official who made a fortune as a real estate developer, told AFP in his museum office in the southwestern city of Chengdu.

But what he calls "political sensitivity" has meant that he keeps the vast majority of his collection hidden from view.

"What I have on display is barely five percent of what I've collected," said Fan, who plans to open a seventh museum on the era next year.

The ruling Communist Party keeps detailed discussion of the Cultural Revolution out of mainstream Chinese media, worried that an open debate could be used to justify unrest and also undermine its official history of a period it refers to as a "serious setback" for the party.

Mao Zedong set the period of lawlessness in motion to boost his authority, previously undermined by the disastrous effort to modernise China known as the "Great Leap Forward," which led to a famine that killed millions in the late 1950s.

China has never stated estimates of how many died in the decade of political campaigns, which saw citizens turning on their neighbours and caused half a million deaths in 1967 alone, according to US-based British historian Roderick MacFarquhar.

The spectacular downfall this year of Bo Xilai -- former party boss of the southwestern megacity of Chongqing, who is set to face trial for corruption and other crimes -- has thrust the Cultural Revolution into the spotlight.

Bo's revival of "Red culture," which saw Maoist quotes sent to citizens' mobile phones and massive "Red song" concerts, along with his charismatic leadership style, reminded many party insiders of Mao's excesses.

China's Premier Wen Jiabao -- lawyers for whom this week rejected a New York Times report on the wealth of his family members -- hit out at Bo's administration in March, when he also called the period "a historical tragedy."

-- "Airbrushing" history --

An increase in social discontent over the past 10 years, evidenced by rising numbers of protests, has made Chinese leaders more reluctant to mention the period, Guobin Yang, professor at US-based Barnard College, told AFP.

"I see a tightening of space for discussion of the Cultural Revolution over the last decade, including on the Internet," he said.

"There is a fear that the Cultural Revolution could be a resource for protesters to justify their activities."

Fan's collections have seemingly escaped censure by mostly avoiding the violence of the time, and by not using the term "Cultural Revolution".

Due to government pressure the period is instead referred to as the more-neutral "Red Era", said a museum assistant, who requested anonymity.

Most government-funded museums in China avoid mentioning the period altogether.

China's National Museum, renovated in 2011, commemorates the era with a lone photograph, and three lines of written text.

"The government's first concern is with keeping society stable, and they know that it would stir up too much criticism to open a museum about the period," Fan said.

"I think it will take at least another 20 years before we can talk openly about the Cultural Revolution."

Fan's museums are part of a growing trend of private museums and galleries being opened in China over the past five years. Of all museums in the country, 13 percent are private, according to the China Daily.

Fan opened his first museum in 2005 in Chengdu and has since expanded to put more of his collection -- boasting more than 100 tonnes of documents including 20,000 diaries -- on display.

Each has a different theme, such as household objects or Mao pin-badges and clocks. Though most of his exhibits avoid the dark side of the 1966-76 social experiment, some do address the violence.

Letters on display in one of the museums tell the story of a Chinese actor who committed suicide in 1967 after prolonged beatings by Red Guards, one of thousands who died during the political campaigns.

But Fan says he is reluctant to exhibit items implicating his fellow citizens in violent crimes "out of respect for their privacy", adding that the items he collected "touch on too many painful memories".

-- Checks on power --

One group that hopes to break the silence are Chinese liberals, who see the chaos as an illustration of the need for democracy and independent checks on the power of the one-party state.

Any mention of the era at China's upcoming party congress -- where a once-in-a-decade leadership transition will be announced -- could be interpreted as expressing the new leadership's commitment to legal and political reforms.

Some commentators have speculated on the basis of recent official statements that "Mao Zedong thought", a traditional part of Communist party dogma, might be dropped altogether, marking a clear break with the era.

"If the Cultural Revolution is referred to in detail at the congress it will probably be as an impetus to push forward political reform," US-based academic Yang said.

But Chinese leaders, who remain focused on stability, are unlikely to make such a reference "unless the new leadership wants the transition to mark a big turning point", Yang said.

Fan, who plans to open a seventh museum about the Cultural Revolution next year, dodges questions about whether the excesses of the period show the need for further political reforms.

"I can't talk too much about these issues, it could bring me all kinds of problems." Fan said. "Above all, I need to preserve my collection."