Zhang Hongbing discusses his role in his mother's death. guardian.co.uk

They beat her, bound her and led her from home. She knelt before the crowds as they denounced her. Then they loaded her on to a truck, drove her to the outskirts of town and shot her.

Fang Zhongmou's execution for political crimes during the Cultural Revolution was commonplace in its brutality but more shocking to outsiders in one regard: her accusers were her husband and their 16-year-old child.

More than four decades on, Fang's son is seeking to atone by telling her story and calling for the preservation of her grave in their home town of Guzhen, central Anhui province, as a cultural relic.

Fang's plot is already hemmed in by buildings and a wall is rising behind it. Nearby streets are stacked with window frames, tiles and pallets of wood. Without official recognition, fears Zhang Hongbing, his mother's grave and story could soon be swept away – part of a wider, shadowed past that is fast disappearing.

"My mother, father and I were all devoured by the Cultural Revolution," said Zhang, 60, who is now a lawyer. "[It] was a catastrophe suffered by the Chinese nation. We must remember this painful historical lesson and never let it happen again."

Zhang Hongbing holds a photo of his mother. Photograph: Dan Chung for the Guardian

Thirty-six million people were hounded and perhaps a million died in the turmoil unleashed by Mao Zedong in 1966. They were condemned by their political views and social background or someone's whim, enmity or attempt at self-preservation through incriminating others. Victims included the father of China's new leader Xi Jinping, who fell from grace and was sent to labour in the countryside.

The Communist party long ago deemed the period a disaster. Even so, authorities are chary of its examination. "It's almost not dealt with at all in official history," said Michael Schoenhals, of Lund University, who co-authored Mao's Last Revolution.

Yet history departments now run courses on the period and there is growing coverage online, he noted. In part, he said, the emerging discussion reflects the passing of time: "The people who were then doing some of the worst things – because they were young and stupid and enthusiastic and eager – are now pushing 70. They want to write before they go, or sometimes their children want them to write it down."

In a chilly study piled high with books and papers, Zhang leafs through family mementoes. One photo records his father being paraded in a dunce's cap. Another shows crudely pencilled illustrations of their story, from an exhibition that lauded Zhang's fervour. In the last sketch, blood spurts from his mother's mouth as she is executed.

The family was once "harmonious, happy and warm", said the lawyer.

Fang, only 44 when she died, was bold, extrovert and honest in all dealings, recalled her younger brother, Meikai. He struggled to speak through his tears: "When I talk about her, I want to cry," he said. "From the age of three I would follow her around; she was like another mother."

Zhang Hongbing's family photos and pictures of his mother. The bottom photo is an original copy of a photo from which other copies had her image torn out. Photograph: Dan Chung for the Guardian

She met her husband when they joined the revolutionary cause, but their life was scarred by politics from the first. Her father was executed as a suspected Nationalist agent; Zhang blames a personal grudge. Later, as they struggled to survive the Great Famine, Zhang's younger brother was sent away to a relative who could feed him.

Then the Cultural Revolution burst into their lives. In the streets of Guzhen, Red Guards smashed heirlooms and burned books: "I thought it was great – an unprecedented moment in history," Zhang said.

In a blaze of enthusiasm, the children changed their names. Zhang, previously called Tiefu, became Hongbing, or "red soldier". His elder sister joined millions of Red Guards trekking to Beijing to see Mao. But shortly after her return, she collapsed and died from meningitis, aged 16. Months later, their father was attacked as a "capitalist roader" in at least 18 "struggle sessions" of verbal and physical abuse.

"I wrote a big character poster about him; I just wanted to follow Chairman Mao," said Zhang. "For a child to criticise their parents wasn't just our household. The whole country was doing it."

In 1968, Fang fell under suspicion due to her father. Two years of investigation, detention and uncertainty tormented her: "Why don't they just make a decision on me?" she asked.

"Her father's death, her husband's persecution, her daughter's death – everything that happened made her suspicious of the Cultural Revolution … She was sick of [it]," said Zhang.

Eventually conditions improved and she was allowed to sleep at home. Then, one evening, her zealous son accused her of tacitly criticising Mao. The family row spiralled rapidly: Fang called for the return of purged leaders and attacked Mao for his personality cult. "I warned her: 'If you go against our dear Chairman Mao I will smash your dog head,'" Zhang said, at times reading from his father's testimony. "I felt this wasn't my mother. This wasn't a person. She suddenly became a monster … She had become a class enemy and opened her bloody mouth."

Fang's brother begged her to take her words back, warning she would be killed. "I'm not scared," Fang replied. She tore down and burned Mao's picture.

When her husband and son ran to denounce her, "I understood it meant death," Zhang said. In fact, he added, he called for her to be shot as a counter-revolutionary. He last saw her as she knelt on stage in the hours before her death.

Most children who turned on their parents were under political pressure, said Yin Hongbiao, a Beijing-based historian.

"Those with 'bad parents' suffered a lot and they resented their parents instead of resenting the system which brainwashed them daily," added Michel Bonnin, of Tsinghua University.

"They were encouraged to denounce their parents, so as to 'draw a line' between them and the enemy. It was the only way to save themselves. There were many cases of children who tried to protect their parents against the violence of Red Guards and were then beaten or even executed."

Zhang's case is much more unusual, but Schoenhals suggested timing was critical: early 1970 saw a harsh campaign against counter-revolutionary activities, known as one-strike and three-anti. "You could come across anything if you had 700 million people embroiled in a conflict of this seriousness and magnitude," he added.

Fang Meikai, though furious with his sister's family, was powerless to help her. "I wanted to see her, but I wouldn't have been allowed. I was afraid that if I went I would also be involved in the case," he said. "That was the situation back then: they could kill whomever they wanted."

Zhang Hongbing: 'I felt this wasn't my mother. She suddenly became a monster.' Photograph: Dan Chung for the Guardian

After Mao's death and the Gang of Four's fall, the political tide turned. Cultural Revolution victims began to be rehabilitated. When Fang Meikai appealed on behalf of his sister, Zhang and his father agreed to support him.

Zhang, belatedly confronting his guilt, said he was a son "who could not even be compared to animals".

Fang was cleared in 1980; two years later, they erected a headstone at her grave, metres from where she was shot. At the execution ground, an acquaintance later told them, her eyes swept the crowd as if looking for faces she knew.

A lost decade

The Cultural Revolution was a lost decade of tragedy and waste. What historians Roderick Macfarquhar and Michael Schoenhals call the "chaos, killing and [ultimately] stagnation" claimed lives throughout the country and at all levels.

Under pressure due to the Great Famine, and unnerved by the Soviet repudiation of Stalin, Mao wielded mass support to see off rivals. Frustrated that Communist ideology had not truly taken root, he also sought to destroy old ideas and institutions.

Top leaders and revered intellectuals were humiliated, beaten and driven to suicide. Youthful Red Guards abused or murdered teachers and bad class elements. In Chongqing rival factions battled with guns and tanks; in Guangxi, there are reports of cannibalism. Friends, neighbours, colleagues and families turned upon each other. Cultural treasures were destroyed; universities shuttered. Millions of "educated youth" were sent to labour in the countryside. The economy was devastated.

Yet many believe that China's reform and development resulted from the era: Deng Xiaoping and other leaders realised only drastic measures could make up for lost time and win back popular support.

Additional research by Cecily Huang