ZHENGZHOU, Henan - China's trade union federation has ordered a nationwide crackdown on labor abuses following gruesome revelations of workers being beaten, starved and forced to slave away in brick yards for no pay.

Grassroots trade unions have been ordered to "immediately carry out a thorough examination to stop the violation of migrant workers' rights," the All-China Federation of Trade Unions said in a statement published by Tuesday in state media.

Federation official Zhang Mingqi was quoted as saying the group was "extremely shocked and indignant" on hearing of the slavery cases.

At least 568 workers have been freed, including dozens of boys, and 168 people detained following media revelations that prompted a series of police raids over recent weeks in the north-central provinces of Henan and Shanxi.

Few details were given in the federation's statement and similar crackdowns have been ordered in the past to little effect. However, the statement adds to signs of high-level concern over the issue, which last week prompted President Hu Jintao and other top leaders to order a thorough cleanup of such abuses.

Typically, slaves were sold to kilns for 500 yuan ($66) each by people traffickers who abducted the young and weak from train and bus stations or off the streets, and lured the stronger with false promises of well-paid jobs.

The use of slave workers came under the spotlight in part because of an open letter posted online signed by a group of 400 fathers appealing for help in tracking missing sons they believed were sold to kiln bosses.

The fathers accused Henan and Shanxi authorities of ignoring them or even protecting the kiln operators. At least one village-level Communist Party chief has been investigated after his son was found to be running a kiln that used slaves.

The fathers' letter said about 1,000 children were being forced to work at kilns under conditions of extreme cruelty, but that number has been impossible to verify.

Authorities have appeared to muzzle further reporting on the scandal after a a flurry of television and newspaper reports last week, including lurid pictures of slave workers with bloody patches of skin burned or rubbed raw. Tuesday's newspapers in Henan's provincial capital Zhengzhou, where the story first broke, contained no items on the slavery, and the local television station did not return calls seeking comment.

Zhang's statement said the crackdown would target industries that typically employ migrants, including manufacturing, construction, mining and restaurants. Unions must unearth abuses and cooperate with local police in rectifying problems, the statement said.

Unions have "failed to play a significant enough role in rural areas," Zhang said.