The police in Beijing have done little to prevent such abuses. They are regularly accused of turning a blind eye or even helping local thugs round up petitioners. That raises suspicions that the central government is not especially upset about efforts to undermine the integrity of the petition system.

The petition system provides people with the semblance of an appeals process that top leaders hope will keep them off the streets. But for officials at all levels, it seems, the appearance of order  measured by reducing the number of petitions  is an acceptable approximation of actual order.

Rights advocates say that black houses have sprouted in recent years partly because top leaders have put more pressure on local leaders to reduce the number of petitioners reaching Beijing. Two of the largest holding pens, Majialou and Jiujingzhuang, can handle thousands of detainees who are funneled to the smaller detention centers, where cellphones and identification cards are confiscated.

China’s petition system originated in the Ming Dynasty, from the 14th to the 17th centuries A.D., when commoners wronged by local officials sought the intervention of the imperial court. Since the Communist Party came to power, the right to petition the central government has been enshrined in the Constitution.

With few legal channels available, petitioners come to Beijing, saying it is their only hope for resolving grievances.

“I know my life is in danger, but I just can’t swallow this injustice,” said Mr. Wang, explaining why he has made 10 trips to Beijing in recent years, each ending in detention.

Image Wu Bowen was confined to this Beijing hotel room for nine days after she sought more compensation for her homes demolition. Credit... Du Bin for The New York Times

Chinese Human Rights Defenders, an organization in Hong Kong that recently interviewed more than 3,000 petitioners, has documented what it says is the lucrative business of abduction and repatriation. “When you’re taken to a black jail, no one knows where you are and you are totally vulnerable,” said Wang Songlian, a researcher.