One night in June last year, Ding Hongfen and about 20 others stormed the Dongjiao Guesthouse in the city of Wuxi in eastern China, breaking down an iron door with crowbars. They freed five people held inside in a “black jail,” an extralegal detention facility operated by the local authorities who regarded the detainees as troublemakers. One of those freed was over age 80, according to the human rights organization that documented the incident.

On Saturday, Ms. Ding and four others, who have been in and out of detention and house arrest ever since, are to be tried in Wuxi on the charge of “destroying property,” according to the organization, Chinese Human Rights Defenders. The “black jail” (in Chinese, hei jianyu or 黑监狱) was one of many such facilities across China where thousands of people are held outside the scrutiny of the law by the local authorities or privately contracted security companies — and where abuse is common.

An important meeting of the Chinese Communist Party’s leadership is under way in Beijing this week — the 18th Central Committee’s Fourth Plenum — which aims to “engrave” the rule of law, Xinhua, the state news agency said. But a report by the rights organization suggests there may be far to go.

About one in seven detentions in the country involves these facilities, whose existence is denied by the state, Chinese Human Rights Defenders said in the report, which was released Tuesday. They are “breeding grounds for horrific abuses,” the group wrote.

Five years in the making, “‘We’ll Beat You to Death With Impunity’: Secret Detention & Abuse of Women in China’s ‘Black Jails’” documents the experiences of 1,000 people who were held in the detention centers inside hotels, guesthouses, residential buildings, psychiatric hospitals, armories, storage facilities, farms and factories. About 80 percent of detainees are women, leading to the focus on that gender. Sexual abuse is widespread, the report says.

The report’s publication also coincides with the 59th session of theCommittee on the Elimination of All Forms of Discrimination against Women at the United Nations in Geneva, which on Thursday is to hear China’s national report on women’s rights.

The Chinese government is “very nervous” about the information about the extralegal jails getting out, said Renee Xia of Chinese Human Rights Defenders.

“It was very difficult to do this report,” she said by email. “The government is very nervous about letting the information about these ‘black’ jails be exposed, for reasons we provided in the report.”

These include their violations of national and international law, and their unpopularity among Chinese citizens. Nevertheless, their use is growing, as the local authorities try to absorb people previously detained in re-education-through-labor facilities, which were closed last year, the report said.

The report was compiled with the help of human rights groups in China whose names were withheld for their protection, Ms. Xia said.

Ms. Xia said that after one meeting with former inmates of black jails and re-education-through-labor camps in Beijing, conducted by the rights group, “police summoned a few of the helpers who gathered the victims and who themselves were victims. Eventually, several of them were detained for months during the crackdown on the New Citizens Movement,” she said, referring to a movement to pressure China’s officials to disclose their assets. Many of the movement’s members have been arrested and sentenced since the beginning of last year.

Many of the inmates of black jails are people who have traveled to larger cities in search of redress for injustices they feel they have suffered in their hometowns. Typically, these petitioners are abducted by “enforcers” hired by their hometown officials who fear the petitioners may embarrass them by revealing local abuses to their superiors. The petitioners are often incarcerated twice, once at the place where they are first seized and then back in their hometowns. Women are more likely to petition over grievances, often on behalf of their families, accounting for the gender imbalance in the black jail population, the group said.

The scale of the system is suggested by the fact that Chinese lawyers and activists compiled a list of 89 facilities in Wuxi alone that they said were being used as black jails, the report said.

It found that while both men and women were beaten and abused in a multitude of ways, women especially were the target of sexual abuse; rape was common, as were being stripped naked and other forms of mistreatment:

Typically guarded by males, women are more likely than men to encounter physical, sexual and verbal abuses and threats. Women are also more likely to be detained and abused for the purposes of intimidating or punishing members of their family.

The women involved are often the poorest and most socially disadvantaged, the report says:

Black jail detainees, such as many whose cases are examined in this report, include some of the most vulnerable women in Chinese society: elderly women, women in fragile health, impoverished rural migrant women, women who lost land or were victimized by forced eviction, women with disabilities, and mothers with their infant or other young children.

Chinese officials deny the existence of black jails. From the report:

“There is no such thing as the so-called ‘black jails.’” Chinese official, at the U.N. Committee on the Rights of the Child review of China, September 2013

The report recommends, among other things, that China release all black jail detainees and adopt “a comprehensive law on violence against women, including a provision about state-sponsored violence.”

And the United Nations needs to demand an end to the practice, the report said:

The Committee on the Elimination of Discrimination Against Women, during its review of China in October 2014, and the Committee against Torture, during its expected review of China in 2015, should call on the Chinese government to fully comply with the relevant international Conventions, to end all extralegal detentions of women and set free all detainees in black jails, hold the perpetrators legally accountable, and provide reparations to victims.

With “rule of law” now on everyone’s lips in China, the rights group recommended:

All U.S., E.U., or World Bank-funded programs intended to push for “rule of law” reform, including trainings of legislators, judges, and police, should emphasize the illegality of black jails, bring up concerns directly with government officials, demanding the dismantling of all extrajudicial detention facilities.