Police in northern China say they have now arrested more than 160 people accused of involvement in slave labour in illegal mines and brick factories.
Among them is the foreman of a brickworks in the northern province of Shanxi, who had been the subject of a nationwide manhunt.
Almost 570 people trafficked as slaves - 50 of them children - have been freed in Shanxi and neighbouring Henan.
President Hu Jintao has ordered an inquiry amid widespread media outrage.
The foreman, 42-year-old Heng Tinghan, is accused of holding captive adults and children as young as eight and forcing them to work long hours for nothing.
The factories were run like prisons with guard dogs and beatings to prevent escapes.
It is thought that many more people are still being forced to work as slaves in appalling conditions, BBC Beijing correspondent Daniel Griffiths reports.
There has been unusually strong criticism in the state-run media with one paper calling the situation a shocking disgrace but the reality is that forced labour is common in rural parts of China, our correspondent adds.
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