BEIJING — Kidnapping villagers who have traveled to Beijing to lodge complaints with China’s central government and keeping them in unofficial jails to silence them has evolved into a cottage industry that police refuse to 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.

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.

However, Foreign Ministry spokesman Qin Gang rejected the Human Rights Watch study and questioned why it was released.

“I don’t know what their motivation was,” he said at a regular news conference Thursday. “I can assure you there are no so-called black jails in China. We put people first, and we are an administration for the people.”

The 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 is 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 have a half-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, the report said. It estimated that Beijing’s black jails detain up to 10,000 people each year.

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 also have “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,” said Nathan, a political science professor at Columbia University in New York.