By Tim Luard
Writer on China
The Chinese characters on the banner at the camp gates say "Virtue Cultivation School". Inside, behind barbed wire and high walls, squads of inmates are herded into the prison factory.
A system of forced labour based on the Soviet gulags was instituted throughout China soon after the communists came to power in 1949, with the emphasis on remoulding "bad elements" into "new socialist persons".
Thousands are held under the "re-education through labour" system
Much has changed in China since then.
But the use of compulsory labour as a means of reform and re-education - both within and outside the regular prison system - remains very much in place.
The purpose of China's network of prison camps and labour farms is brainwashing, according to Bob Fu, a pastor who was detained for "illegal religious activities".
"The prisoners have no free time to think independently," he says.
"They get up at 0500 and work all day, sometimes until midnight. There are also political study sessions. But the main idea is to solve your 'mind problems' through hard physical labour."
Another former prisoner said he had laboured "as a slave" for 14 hours a day and was beaten if he could not complete his quota.
"We carried stones to a river wharf all day then made artificial flowers at night, seven days a week", says Chen Pokong, a teacher and pro-democracy activist who spent two years in a labour camp in the mid 1990s.
He is now studying in the US but says he still knows people in the camps who tell him little has changed.
Using their vast pools of free labour, China's prisons produce everything from green tea to coal, paperclips to footballs, medical gloves to high-grade optical equipment.
They have played a key role in the country's dramatic economic rise by processing goods for export, according to Harry Wu, a prominent US-based campaigner who was held in labour camps for 19 years.
"But the basic system remains what it always was - a tool to help a totalitarian regime maintain control," he says.
Some of Mr Wu's claims have been criticised by other observers as exaggerated or out-dated.
His estimates of the total number of inmates have ranged from four million to 20 million. But many believe the true figure today to be around two million - which is fewer than the number behind bars in the United States.
As for using "slave labour" to conquer the world's markets, prison goods are in fact thought to play a very minor role in China's exports - if only because their quality is too low. While some prison officers may be lining their own pockets, the system as a whole runs at a loss.
Prison labour, moreover, is not in itself against international law.
What really concerns international rights groups are the often appalling conditions in the camps and the arbitrary way in which people are sent there.
Most inmates in the regular prison system have been convicted of what would generally be regarded as crimes in the West. And only a small proportion these days are political prisoners (China denies it holds any at all).
China's prisons produce a variety of goods
But anyone can be held under a form of detention known as "re-education through labour" with no trial or sentencing procedure of any kind.
This system allows police to send people to labour camps for up to four years on a variety of vaguely-defined offences without having to present a case to prosecutors or judges.
About 300,000 people - the highest number ever - are currently held under this system in some 300 camps.
The wide police discretion has made re-education through labour a key weapon in crackdowns against unauthorised religious groups and separatist-minded ethnic minorities.
Other targets include drug addicts, prostitutes, "hooligans and lazy people" and even, according to one regulation, people who have extra-marital affairs.
Falun Gong, the spiritual movement banned as a cult, says tens of thousands of its followers have been held in these camps, with many subjected to torture. More than 1,000 of them have died, it says.
Human rights groups have been unable to confirm those deaths but say there are frequent reports of physical abuse by guards using electric batons and by cell bosses selected to "maintain order" over fellow inmates.
Meg Davis of Human Rights Watch, who visited a labour camp for drug-users in the south-western province of Yunnan, described it as unclean and seriously overcrowded.
"It was also highly militarised like something from the Mao era, with inmates marching round chanting slogans," she said.
Sweatshops were turning out fake diamonds for sale to tourists, she said.
Some of the harshest conditions are found in the labour farms and prison mines in
remote western regions such as Xinjiang, according to James Seymour, co-author of New Ghosts Old Ghosts - Prison and Labor in China and a research scholar at the Chinese University of Hong Kong.
"Chinese mines in general are notorious for their poor working conditions and frequent accidents. And there's evidence that the prison mines are even worse than the civilian ones," he said.
But the biggest problem, he believes, is that it is often the wrong people who are detained in the first place.
The Chinese government is now debating legislation to reform the re-education through labour system.
"The power to send someone to prison should belong to a court," Xu Zhiyong, a Beijing law professor, told BBC News.
The proposed reforms could happen by the end of this year, he said, but face opposition from certain departments. Whatever happens, it is likely to be old wine in a new bottle.
The lack of any internal checks on abuses or access by inmates to lawyers mean China will still to fail to meet international standards on detention and forced labour, rights groups say.
And it does not help that the whole subject is a state secret.
"The key issue for us is that the prison/detention facilities are a black hole", says Nicolas Becquelin, research director of Human Rights in China.
"Nobody can visit them, not even the International Red Cross. Governmental delegations are taken on potemkin tours that don't mean anything. There are a lot of reform efforts going on, but without independent access self-medication will not cure the system".