BEIJING — It is hard to say exactly which “subversive” sentiments drew the police to Ren Jianyu, who posted them on his microblog last year, although “down with dictatorship” and “long live democracy” stand out.

Last year Mr. Ren was among tens of thousands of Chinese who were dumped into the nation’s vast “re-education through labor” system, a Stalinist-inspired constellation of penal colonies where pickpockets, petitioners, underground Christian church members and other perceived social irritants toil in dismal conditions for up to four years, all without trial. With as many as 190,000 inmates at any one time, it is one of the world’s largest systems of forced labor.

But now the labor system, known by its shorthand, “laojiao,” is facing a groundswell of opposition from both inside and outside the Communist Party. Critics say the once-in-a-decade leadership transition last month, which included the demotion of the chief of the nation’s vast internal security apparatus, has created a potential opening for judicial and legal reform.

“It’s high time we demolish this unconstitutional and abusive system that violates basic human rights, fuels instability and smears the government’s image,” said Hu Xingdou, a professor at the Beijing Institute of Technology who frequently rails against the system that Mao Zedong created in the 1950s to take down suspected class enemies and counterrevolutionaries.

The calls for change go beyond longstanding advocates of political reform like Professor Hu. China’s national bar association is circulating an online petition that has been signed by thousands. Legal experts have convened seminars to denounce the system. And almost every day, it seems, the state-run news media, with the top leadership’s tacit support, report on hapless citizens ensnared by the arbitrary justice that the local police impose with the wave of a hand.

Mr. Ren’s case would probably have gone unnoticed if not for China’s increasingly emboldened human rights defenders, who showcased his plight on the Internet. Evidently prodded by the torrent of news coverage, Chongqing officials cut short his two-year sentence and freed him.

“It was a depressing, dreadful experience,” Mr. Ren said in a telephone interview this month, describing long days spent in the camp’s wire-coiling workshop.

Other examples abound. A migrant worker from Inner Mongolia was sent away for quarreling with an official at a restaurant. A mother from Hunan Province was given an 18-month sentence after she publicly protested that the men who had raped and forced her 11-year-old daughter into prostitution had been treated too leniently.

This month an 80-year-old Korean War veteran with Parkinson’s disease sobbed on national television as he described spending 18 months in a labor camp as punishment for filing local corruption complaints.

People’s Daily, the Communist Party’s mouthpiece, took aim at the system last month, saying it had become “a tool of retaliation” for local officials. In October the head of a government judicial reform committee noted a broad consensus in favor of addressing the system’s worst abuses.

And in a widely circulated recent essay, the vice president of the Supreme People’s Court, Jiang Bixin, argued that the government must act within the law if it is to survive. “Only with constraints on public power can the rights and freedoms of citizens be securely realized,” he wrote.

China’s incoming president, Xi Jinping, has not yet weighed in on the issue, but reform advocates are encouraged by a speech he gave this month talking up the widely ignored protections afforded by China’s Constitution, which include freedom from unlawful detention and the right to an open trial. “We must establish mechanisms to restrain and supervise power,” Mr. Xi said.

Until now, China’s powerful security establishment has staved off any erosion of its authority, warning of calamity if the police lose their ability to detain perceived troublemakers without the interference of judges or defense lawyers.

The Ministry of Public Security has other reasons to preserve the status quo. The system, which employs tens of thousands of people, is a gold mine for local authorities, who earn money from the goods produced by detainees. Officials also covet the bribes offered to reduce sentences, critics say, and the payments families make to ensure a loved one is properly fed while in custody.

Zhou Yongkang, who retired last month as the chief of domestic security, was known to scare senior leaders with the specter of social chaos — and the party’s loss of power — if the extralegal penal system were abolished.

“Zhou Yongkang knew how to put on a good show,” said He Weifang, a Peking University law professor who has waged a decade-long campaign against the security system.

Professor He and other legal experts said they were encouraged after party leaders downgraded Mr. Zhou’s law-and-order responsibilities, moving them to the 25-seat Politburo from the more influential Standing Committee.

The change underscores the leadership’s increasing wariness of China’s internal security machine, which grew by leaps and bounds in the decade under President Hu Jintao’s campaign for “social stability.” The annual $110 billion security budget now exceeds China’s military spending.

“There’s a sense that things have gotten out of control, that when there’s an intraparty struggle, even the most powerful official can be disappeared,” said Sharon Hom, executive director of Human Rights in China. “No one is immune from a lawless state.”

Guo Xuehong, a former judge from Jilin Province in the northeast, has firsthand experience with extralegal justice. Mr. Guo, 47, was given a one-year sentence after protesting his dismissal — a result, he said, of refusing to back down from his ruling against a politically connected company. His ruling harmed the local economy, party officials determined.

In an interview, Mr. Guo complained that those ensnared have no ability to mount a defense and little chance of appealing their sentences, given that they are reviewed by the same agency that metes out summary convictions.

“The system is just a tool for officials to get rid of us troublemakers,” Mr. Guo said.

Mr. Guo was lucky enough — and perhaps well connected enough — to be allowed to serve part of his time at home because of poor health. Those who have spent time in one of the nation’s 350 labor camps describe inedible food, overcrowded cells and brute violence.

The vast majority of inmates are petty criminals, but political offenders and underground members of the banned spiritual group Falun Gong are often singled out for the worst abuse, especially if they protest the conditions or refuse to confess.

Liu Jie, 60, a former business executive from Heilongjiang Province, served a two-year sentence for “disturbing social order,” punishment for releasing a public letter demanding political and legal reform. She described how roughneck inmates violently imposed the guards’ will in return for reduced sentences. One particular thrashing cost her several front teeth and left her temporarily blind in one eye.

When she complained about dizziness from the paint used to make paper lanterns, Ms. Liu said, she was hogtied to a chair for a week, a dreaded punishment known as the “tiger bench.” Deprived of food and water for several days, she said, she repeatedly lost consciousness. “When they unshackled me from the chair, my legs had turned black with bruises.”

In a 2009 report,  Chinese Human Rights Defenders documented what it called a “hotbed of injustice,” with inmates sometimes working 20-hour days to produce chopsticks, firecrackers, cardboard boxes or handbags.

“To meet the quota, we had to work so hard our fingers became coarse and swollen, with little blisters on top of our big blisters,” one detainee told investigators. Inmates are paid nothing or at most the equivalent of a few dollars a month.

Ms. Liu said, “It’s a big sweatshop built on an enormous chain of profits.”

Rights advocates say a genuine overhaul of the laojiao system would require, among other things, allowing victims access to lawyers and the right to appeal. But many of them fear that party leaders may instead opt for only modest modifications. Recent trial balloons include limiting sentences to a few months, providing inmates with weekend furloughs or requiring the police to issue written verdicts.

Such changes, said Professor He of Peking University, would accomplish little. “Technically speaking, laojiao is easy to fix,” he said. “But the danger is that all we’ll get is a deceptive name change.”