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.

In the end, Mr. Ren, 25, a college graduate from Chongqing, the southwestern metropolis, was sent without trial to a work camp based on the T-shirt that investigators found in his closet: “Freedom or death,” it said.

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.