The faces of China’s lost boys stare out from hundreds of pictures that once captured joy but now serve only to remind their parents of a cruel loss of innocence.

A plague of kidnapping has swept across Yunnan, a remote southwestern province, claiming hundreds of boys from the city of Kunming alone.

One vanished while his father bought sweets. Two more were led away in broad daylight from a busy market. The children have gone from poor townships and rural farms. They are the sons of China’s working class but their fate has been covered up on the orders of the Communist party.

“As of the end of 2004 there were more than 200 boys missing from Kunming city alone,” said Lu Youmin, a businessman, who has emerged as leader of a group of bereft parents fighting for action.

The motive is greed. Gangs of traffickers snatch the children to sell to childless couples in the prosperous cities of coastal China, where they will be passed off as “adopted”.

Lu is an exception. The kidnappers took his daughter Lu Shanni, then aged eight, in 2002. But 95% of the stolen children are boys, prized because they will carry on the family line.

It is a well organised trade that the state is plainly unable to stop. Official propaganda highlights the occasional police success. Yet there is no helpline, no nationwide appeal, no television broadcast with pictures of the missing.

Instead, a Sunday Times inquiry has found that police have refused to investigate cases and have harassed families who dared to complain. The official preoccupation with silencing publicity about the kidnappings may stem from embarrassment.

A United Nations report shows that three years ago the authorities were warned about the surge in boy abductions. They have done little, if anything, to curb it.

Last week as Sha Zukang, the Chinese ambassador in Geneva, was defending the country’s record on child protection before a UN committee, police in Kunming arrested parents who tried to contact this newspaper for help in searching for their missing sons.

Some were infants, some toddlers, some were taking their first rides on swings or in plastic cars before they were abducted. The Sunday Times has seen the names, photographs and other details in more than 60 cases. All remain unsolved.

Most of the victims were poor. They were the sons of impoverished migrant workers who exist at the bottom of the heap in China with neither the money nor the political connections needed to get help.

Most of the perpetrators, the people who buy the boys, are wealthy. They can pay up to £3,500 for a child. They are the shame-free rich class of post-revolutionary China.

The pain inflicted by their selfishness is written on the face of Pu Caiju, 32, whose son, Li Shang, was four when he vanished outside the family’s village home on March 14, 2002.

She and several other parents risked official wrath to be interviewed in central Kunming. “My husband had been playing with our son and he said, ‘Just stay here while I go upstairs for a cold drink’. He was away not five minutes but when he came back our son was gone,” she said.

Sixty villagers joined the hunt, scouring the bus station, the alleys, the back yards. Then they went to the police.

“The police just weren’t interested,” said Pu. “They told us to come back if we hadn’t found him in 24 hours. We suspected that a neighbour, who was a drug user, may have been involved but the police didn’t want to know.”

Pu’s husband went all over southern China to try to find their son. Then he had a breakthrough. In a rare, well- publicised success, the police had caught a gang of 11 child traffickers. “My husband was allowed to confront one of the suspects with a photograph of our son and the man said, yes, he had sold him very easily because our son was so smart,” Pu said.

But the parents’ hope that the traffickers could lead them to the missing children was abruptly cut short after their trial and conviction. Keen to impress the public and central government with their zeal, the authorities swiftly executed nine of the 11, including the man who had confessed to selling Pu’s son.

“The police had found out very little before they were executed,” Pu said. “All the families were horrified.”

If Pu, like many Chinese people, conceals her emotions behind a mask, that effort must have been doubly difficult for the woman who sat next to her.

Luo Qin, 28, lost her two sons, Wang Tao, then aged eight, and Wang Wei, then five, on October 1, 2003.

“I was at work, so was my husband, and the boys were playing with a neighbour’s child,” she said. “When my husband came home, another neighbour said the three had been just outside 15 minutes earlier — but they had gone.”

The third child, Pan Kunkun, aged five, was also abducted. The families went to the police only to be met with the same response. “They said, ‘Don’t bother us. Come back in 24 hours if you can’t find them’,” said Luo.

She and Pu were asked whether they would think about having another child. They shook their heads in unison. “We don’t want other children. We want our children back,” Luo said. For all the parents a lost child seems frozen in time. They speak of their sons as if they were still the same age as on the day they disappeared.

None wants to face the reality that the infant they nurtured may be growing up in a more comfortable home, far away, forgetting his real family.

Ai Feng Xin, a 40-year-old vegetable seller, shares Luo’s double burden of guilt. His two sons, Ai Yu and Ai Qin, aged four and two at the time, vanished from a Kunming market in the few minutes he needed to park his three-wheeled vehicle.

“We suspected a man in the market but the police let him go. First they wouldn’t register the case properly. Then they wouldn’t give me a blood test for identification because I didn’t have the registration papers,” he said.

Ai is so angry that he shouts answers to questions and has no fear of the police. “I suspect some of them are in league with the smugglers,” he charged.

The one-child policy enforced by the state complicates the issue. It penalises the parents who have lost their boys and gives a huge incentive to couples prepared to do anything for a son.

It imposed a harsh irony on Reng Zhongquan, 38, a quietly spoken tailor, who already had a teenage daughter when his wife found she was pregnant with their son, Reng Pan.

“We wanted to keep him, of course, so we paid the fine of 10,000 yuan (£696) and got him official papers,” recalled Reng.

Reng Pan vanished on September 9, 2003, aged five, while his father was buying sweets at a stall one minute’s walk from their apartment.

“We called the police emergency number but nobody came. Eventually they registered the case but they wouldn’t even question the neighbours,” Reng said. “Now we families must stick together.”

The families did stick together. Even as victims of crime, it was a risk to take. So when Lu discovered that hundreds of families shared his grief, he was surprised to find out at the same time that their rage transcended their fear of offending the state.

They united to lobby the Kunming police for action. They pleaded with Chinese reporters to publicise their losses. In the end they did what Chinese dissatisfied with local officials have done since imperial times: they spent hard-earned savings to travel to Beijing in order to take their complaint to the seat of government.

The response was a standard mix of propaganda, blandishment and threat. The police announced arrests. The press dutifully recorded the crackdown. The problem was declared to be in hand. The families were told to stop disturbing everybody.

There was, it seems, not enough manpower to devote to a serious search for the boys. Yet when Kunming hosted a pompous “summit” for business executives last summer, the police found sufficient resources to go door to door to warn the parents against causing any public embarrassment.

Nobody could claim the Chinese authorities had not been alerted to the danger posed by the kidnap gangs years ago.

In 2002 the local police, security officials and social services all co-operated in a report by the UN’s International Labour Organisation into people trafficking in Yunnan. It estimated that 1,000 women and children were snatched every year from the province.

“Since the 1980s, trafficking in women and children in China has grown at an alarming rate,” the report found. Most stolen infant boys were sold for adoption, it concluded: “In many cases the victims have never been heard from again.”

The report warned that traffickers were increasingly resorting to kidnapping because it was becoming more difficult to find peasants ready to sell a child. It described a chain of criminals who bought and sold children. It spoke of girls forced into prostitution and small boys being trained as beggars. Officials estimated that 338 organised human trafficking gangs were at work in Yunnan, where 43m people live.

The UN recommended a government campaign to raise awareness, monitor vulnerable migrant districts and increase the punishment for those who buy children from the current maximum of three years in jail.

Last week the Kunming police were certainly on the alert. On Thursday morning Ai Feng Xin, Pu Caiju and the husband of Luo Qin were on their way to join a dozen parents willing to tell their stories. They and three others were arrested as they left their homes. The police ordered them not to speak to foreigners and forbade them to gather together. They were detained until the evening.

On Friday, one of the parents called from a payphone that could not be monitored by the police. The message was simple. “Thank you for coming to listen to us,” the parent said, “We thought nobody cared.”