More than 1,000 child slaves may have been maimed, starved and brutally beaten while being forced to do physical labor in primitive brick kilns in Central China, the state media reported Friday.

The disclosure came as the China Daily reported that nearly 500 slave laborers working in kilns and coal and iron ore mines had already been freed during a sweep of Henan and Shanxi provinces by 35,000 police officers.

The state's communist paper reported that a four-day crackdown by police had freed 217 slave workers in Henan Province, including 29 children, and detained 120 suspects.

In the area around Xinxiang, north of Zhengzhou, police raided 20 brick kilns on Saturday and rescued 23 people, including 16 children. Another 251 workers, including 80 boys, were freed in Shanxi, according to Xinhua and China Central Television.

Click here to read the People Daily's story.

Guarded by taskmasters and dogs, the slave laborers were forced to work 16 hours a day and finish their meals of steamed bread and water within 15 minutes. The workers slept on the ground in a dark room without a heating system in the freezing winter. One child was accidentally killed with a shovel, according to reports.

At least 13 workers died of overwork and abuse in Shanxi, CCTV said, citing the provincial Public Security Bureau.

The slave laborers had been lured by human traffickers looking to staff brick kilns that had fallen into debt and could not afford to employ local workers.

A typically tight-lipped Chinese media discovered the story of the slave laborers in the past two weeks, exposing the plight of the captive children and printing photos of distraught parents. As many as 400 Henan fathers reportedly traveled to the remote mountains in Shanxi to track down missing sons who they believe were sold to kilns.

"We must do everything we can to fight human trafficking and rescue those being held captive," said Qin Yuhai, vice governor and police chief of Henan.

Wang Bingbing, owner of an illegal brick kiln, and four accomplices were detained after police found they had forced 32 people who had been abducted or lured from railway stations in Henan and Shanxi. Heng Tinghan, the foreman of a kiln in Hongtong, is listed by the public security minister as a wanted person of the second-highest importance.

Nine of the 32 were mentally disabled, and one was beaten to death last November, local police said.

One mother, Yang Aizhi, went in search of her 16-year-old son who disappeared March 8. She'd heard of children who'd been kidnapped and forced into labor at the Shanxi kilns. She visited more than 100 kilns and saw "most kilns were forcing children to do hard labor," she was quoted as saying in the Southern Weekly. Some children were still dressed in school uniforms and were whipped when they were too tired to push carts.

Yang tried to rescue some of the children but was threatened by kiln owners. She hasn't yet found her son.

The raids were prompted in part by an open letter posted online and signed by a group of 400 fathers appealing for help in tracking down missing sons they believe were sold to kiln bosses.

The fathers accused Henan and Shanxi authorities of ignoring them or even protecting the kilns and human traffickers, saying about 1,000 children were being forced to work at kilns under conditions of extreme cruelty.

The letter sparked an outpouring of television and newspapers reports, along with widespread discussion on the Internet. In just one such report Friday, Shanghai's Oriental Morning Post ran a large photograph on its front page of one of the slaves in Shanxi's Hongtong county, showing his skin rubbed raw and bloody.