Kidnapping people on their way to lodge complaints with China's central government has evolved into a lucrative cottage industry that mainland police refuse to acknowledge or crack down on, a human rights group said Thursday.

The report by New York-based Human Rights Watch on China's "black jails" is based mainly on interviews with 38 people who said they were nabbed by thugs while trying to bring grievances to the central government. They reported being held for days or months in makeshift detention centers, deprived of food and sleep, beaten and threatened. Police allegedly aided the captors or refused to intervene in several cases, it said.

China's Ministry of Public Security refused to look at the 53-page online report in English and requested a summary in Chinese. In response to a summary prepared by The Associated Press, a spokeswoman said the ministry was not responsible for any alleged violations and could not verify secret jails exist in China. She refused to give her name in line with ministry policy.

Black jails emerged in China about six years ago after police were barred from randomly detaining vagrants. The jails, usually makeshift lockups in hostels, apartment buildings, or abandoned factories, have been well-documented by human rights groups, lawyers, and the international media.

The HRW report sheds new light on the economics of the jails and why they evade crackdowns despite violating Chinese and international law.

It blames a civil service evaluation system that uses a point system to penalize officials if too many people from their jurisdiction complain to the central government and rewards those who are able to minimize grievances. Because bonuses and promotions are linked to evaluations, it's become economical for officials to pay people to intercept, detain and intimidate petitioners, it said.

The report cites an alleged internal government directive given to authorities in Shimen, a county in south China's Hunan province, in 2007 that says officials get two points if they bring petitioners back from Beijing or the provincial capital of Changsha, while those who fail to do so are to have half a point deducted.

Officials typically pay black jails between 150 yuan ($22) to 300 yuan ($44) per day to hold petitioners until they can be picked up and returned home, it said, and estimated that Beijing's black jails detain up to 10,000 people per year, though that number includes some people who are detained on multiple occasions.

Police in Beijing and other cities are aware of the jails but ignore them because they keep potentially troublesome petitioners away from cities, Human Rights Watch said. In some cases, police have also "directly assisted black jail operators," it said.

"It's completely illegal but the national authorities have done nothing to stop it so far," said Andrew Nathan, an expert on Chinese human rights issues who was not involved with the report.

"At the same time, though, this informal system cuts against the ability of the central authorities to learn about what's going wrong at the local level," he said. "In the long run it would be smarter for Beijing to let the petitioners exercise what are after all their legal rights."