More than 45,000 police fanned out across Henan and Shanxi provinces to crack down on instances of forced labor. The investigation was prompted, in part, by an Internet campaign mounted by hundreds of parents who said their children have been kidnapped and sold to brickyard owners.

Revelations about the extent of the abuses, which received wide coverage in state-owned newspapers and on television news, come at a sensitive time for Communist Party leaders, who are looking to polish the country's international image ahead of the Olympic Games in Beijing.

Propaganda officials in recent days have begun trying to rein in media coverage, especially on the Internet. Instructions from the government called on Web sites to restrict comments on blogs and online forums, and some information began disappearing from Web sites over the weekend.

Xinhua, the government-run news agency, reported that workers, some of them children, had been kidnapped or lured to work at the brick kilns with false promises and then held against their will and forced to work long hours without pay or adequate food. According to Xinhua, a total of 168 people have been arrested in the forced-labor sweep.

Han Dongfeng, a Hong Kong-based labor-rights activist and founder of the China Labor Bulletin, says that forced labor has been a problem for more than a decade, especially in brick kilns, coal mines and small garment factories.

Mr. Han credits the online campaign by the parents of missing children with prompting the authorities to take action on an issue long swept under the rug.

The Communist Party's main national newspaper, People's Daily, carried a lengthy article saying that police had stumbled by accident on slave workers at a brick kiln owned by the son of a village Communist Party boss in Shanxi province at the end of May.

The party official's son, Wang Binbin, was quoted by People's Daily as saying he began employing workers provided by human traffickers after falling into debt and being unable to afford to pay locally recruited laborers. Workers were severely beaten when the pace of their work slowed, the report said.

Mr. Wang and two others were arrested and 31 workers freed by the police in that case.

To sign up for The Wall Street Journal Briefing please visit http://www.briefing.wsj.com/hktdc.