Behind the glitz of the Beijing Olympics which begins this week China hides a shameful secret — up to 20,000 children are being sold on the black market each year.

The secret trade is booming due to China's controversial one-baby rule introduced in 1979 to curb a population explosion.

Parents must have a birth permit to have a child. If they don't and they are discovered, they are fined. If couples have a second child, they have to pay a fine.

Many poor families can't afford to raise a child, pay the fees and feed themselves, so they feel selling their child is the only way to survive, but even more children are kidnapped.

While China tries to focus the world’s attention on the Olympics, thousands of families are still desperately trying to trace their children who have been sold to strangers.

The Chen family’s son, five-year-old Chen Jie, disappeared after helping his grandmother on her vegetable stall. He was being taken home by a neighbour, Zhang, who is believed to have sold him. The going price for a boy on the black market is around £650 — six months' wages for an average Chinese worker.

Chen Jie's father Chen Lung, a plasterer who lives with his wife Li in a tiny flat in the migrant workers' ghetto of Kunming, Yunnan province, has spent the past two years searching for him.

The couple have notified police, joined a support organisation made up of other parents of missing children, plastered posters on lampposts and hired a private detective.

"Someone who has never lost their child will never understand this kind of pain," Li said, tears streaming down her face.

SHIVERING

"It is like a knife through my heart. When it's raining, I wonder if he is getting wet. When it's cold, I wonder if he is shivering. I don't know how he is feeling. Is he calling someone else mummy and daddy? Is he dreaming of us?

“I used to joke when he was naughty that I'd send him away or sell him. Now that he's gone, he probably thinks that I meant it. He'll be saying: ‘Mummy, I'll be good, I'll be good’."

His father added: "It's torture. We don't know if our child is dead or alive. Is he being fed, is he being beaten, is he being looked after, is he at school?"

More than a year ago Liang Di (3) went missing while shopping at a market in Dongguan with his father and brother.

The two boys were playing outside while their father was in a hardware store. A man approached the two brothers on the street and offered them sweets. Then the man took little Di away.

His parents went to the police but were told that they needed to find more clues before the authorities could do anything to help. The police never launched an investigation. Father Liang Xiangrong said trying to solicit help from government officials is useless.

"We think of him every day. When we close our eyes in bed at night, we think of him. When we see other people's children, we think of him. We really miss him," Liang said.

The 2007 US State Department Trafficking in Persons report says that domes

tic trafficking "remains the most significant problem in China".

It estimates that there are up to 20,000 victims each year. Some estimates put the number of children kidnapped or sold on the black market closer to 70,000.

Wai Ling (19) and her boyfriend (21) are too young to get married as per Chinese law. They had a daughter and without being married they couldn’t register their daughter's birth asthe law would impose a heavy fine on them.

They had only two options — either bribe to a state official or sell the baby to a baby trafficker. With a small salary they couldn’t afford the former, so they had to go for the latter.

"Every mother assumes she will be with her child for ever. But soon she will be growing up in another family. As a mother, I really want to watch her grow every day with me. But I know that's not possible,” Wai Ling said.

The Single Child Policy was introduced in 1979 as a bold attempt to stem China's population. But, instead of becoming a solution, it has created a bigger problem.

Some of the kidnapped children are forced into work, girls often into the sex trade. Others are purchased by families who desperately want a child, usually a boy, to carry on the family name. But there is also a growing concern that some make their way overseas, with unsuspecting adoptive parents who don't realise that some orphanages have baby-buying programmes, offering cash for children.

In 2005 there was a child-trafficking crackdown in Hunan province. Six orphanages were caught purchasing infants from traffickers who had transported them from Guangdong province.

In three years nearly 1,000 children were purchased by these six orphanages, which then adopted the children out to domestic and foreign families for a profit.

Twenty-seven people were arrested, 10 of whom were jailed. Despite this warning to offenders, the baby-buying practice seems to be continuing in the same province.

Liang Di's parents have no idea where he is, but the prospect that he may have been adopted offers no comfort to them.

"However hard our life is, we would never want the boy to be adopted. We wish we could have him back," his father said.

VALUED

In rural China, boys are still valued over girls. Boys are seen as having more earning potential and the ability to carry on the family name. Many families are so eager to have a boy, they are willing to buy one on the black market.

It is a criminal offence in China to kidnap women and children. It is also illegal to abandon, sell or buy a child. The Chinese have implemented new programmes aimed at prosecution and prevention, but aid agencies say the policies don't go far enough.

As a result more couples are suffering like Hu and He Yang. Five years ago their son Huang Yang Yang was snatched from outside their home. Since then they have heard nothing.

Hu and her husband went to the police, who took two days to make a crime report — and never searched for him.

When Hu discovered that a man who used to live down her street had received the death penalty for his involvement in child trafficking, she confronted his wife.

She said: "I thought she would lead me to the gang who stole my son."

But Hu received a savage beating from a group of thugs because of her inquiries. "They told me to stop following her. I was punched and kicked unconscious. I went to the police with the information but they did nothing. Then I got an even worse beating when news filtered back about going to the police.

"Four men cornered me close to my home. I got stabbed five times in the ribs. My husband found me unconscious in a pool of blood.

"They left me for dead but my husband and I would gladly trade our lives for information that would lead to the return of my son."

Three years ago He and Hu — still searching for their son — were visited by officials who had discovered the couple had four children. They were dragged to a clinic where eight men held He down as a doctor performed a vasectomy without an anaesthetic.

Afterwards he was dumped on the street. His wife was lying sobbing on the ground next to him. Villagers carried the couple back home and officials simply crossed their names off a list and moved on to the next victims.

Human rights violations like this are just one reason campaign group Amnesty International say China should not be allowed to host this year's Olympics.