Forty years after his death, Mao Zedong's presence remains impossible to escape in China, yet difficult to discuss.

His corpse still lies in state in the centre of Beijing, watched over by a giant portrait hanging on the Forbidden City in Tiananmen Square.

His face peers out of every wallet, emblazoned on the bank notes that have powered his country's rise to the world's second largest economy.

Yet his legacy remains problematic for China, the Communist Party, and its current chairman Xi Jinping, according to Frank Dikotter, a Mao expert at Hong Kong University, describing him as "both the Lenin and the Stalin" of the ruling party.

"He's both the one who like Lenin brought the Communist Party to power and he's the one who like Stalin committed horrendous crimes against humanity," he said.

The son of a wealthy farmer, Mao dreamed of transforming the nation into a communist paradise and stopped at nothing to achieve his vision.

He was among the Chinese Communist Party's founders in 1921, and fought for 28 years against his own countrymen and the Japanese.

Finally on October 1, 1949 he declared the People's Republic in Tiananmen Square, but his dream quickly turned into a nightmare.

He ordered multiple purges to fight "counter-revolutionary" influence in the party, which are believed to have killed hundreds of thousands of people.

Tens of millions starved to death in the late 1950s during his Great Leap Forward, an ill-conceived attempt to force the country into communes.

And in the decade leading up to his death, he unleashed the Cultural Revolution, an orgy of spiritual and physical violence that deeply scarred the national psyche.

Looking back on that history, the ruling party issued a 23,000-word resolution describing Mao as "a great Marxist and a great proletarian revolutionary, strategist and theorist" who made "gross mistakes".

The verdict is often summarised as "70 percent right and 30 percent wrong", a stance that hasn't really shifted even as the reforms instituted by Mao's successor Deng Xiaoping transformed the country, Dikotter said.

"You can't really touch upon the whole credibility, reputation, image of Mao, without undermining the foundation of the Communist Party of China."

- 'Sanctioned amnesia' -

Under President Xi, the government has gone to ever greater lengths to make sure everyone says the same thing when they talk about Mao.

The most powerful leader since the Great Helmsman himself, Xi has cautioned against both "historical nihilism" and "neo-liberalism", an implicit warning to bury both praise and criticism of Mao's era.

"There is an officially-induced and sanctioned amnesia about Mao's true record," said Fei-Ling Wang, a China expert at the Georgia Institute of Technology.

Criticism of his failures may undermine the government's legitimacy, and last year a Chinese television anchor was suspended after a video surfaced of him criticising the dead leader at a private gathering.

But praising his ideology can also be seen as a lament to how far down the capitalist road the country has gone: Chinese officials in January ordered the demolition of a giant, golden statue of the Chairman just days after pictures of its construction appeared online.

"Citizens, artists, and activists all have to navigate the grey and shifting boundaries of what's politically permissible," said Jessica Chen Weiss, an expert on Chinese politics at New York's Cornell University.

- Better than Jesus -

For many, Mao's legacy remains highly subjective, said Jeff Wasserstrom, editor of the Oxford Illustrated History of Modern China.

An unemployed steelworker might think of "a more heroic, to him, Mao of the mid-1950s who spoke of labourers being the natural 'masters' of society and promised men like him... jobs for life", he said.

Meanwhile, victims of the Cultural Revolution would see "a doddering figure who made bad decisions that plunged the country into chaos", Wasserstrom said.

Some Chinese retain an almost God-like reverence for Mao, similar to the cult of personality that once surrounded him, said Li Yaxing, a professor of Mao Zedong Thought at Xiangtan University in the ruler's home town.

"No one is perfect. Even the Cultural Revolution was a mistake he made while discovering the road of socialism with Chinese characteristics," she said.

It was hard to find anyone comparable, she said, adding: "Even Jesus didn't enjoy such a high reputation."

For Dikotter, China's top leaders' relationship to Mao is more personal than reverential.

For them, the chaos of his era is akin to a family secret: "Most of the leaders and their families were involved in it, including the family of Xi Jinping," he said.

"All party members have a stake in making sure that history is rarely scrutinised," he added.

"All of them have a stake in making sure Mao's portrait stays up there."