Authorities have freed another 80 slave laborers who were starved, beaten and forced to work 14 hours or more per day at brick kilns and small mines in central China, state media said Saturday.

Police in Henan and Shanxi provinces have staged a dayslong series of raids on kilns and small coal and iron mines, acting with a rare speed and thoroughness indicating high-level government concern.

The newly freed workers bring the total of slave laborers rescued in the past month to 548, the official Xinhua News Agency said.

Shanxi province has mobilized more than 14,000 police officers to raid some 2,500 kilns and mines, Xinhua said, citing Du Yulin, director of the provincial public security bureau. Authorities have been given 10 days to investigate every facility and interview each worker, Du said.

Dozens have been detained or arrested, Xinhua has said.

Media reports have said workers as young as 8 were recruited from bus and train stations with false promises or abducted off the street, then sold to kilns for $65 each.

Chinese President Hu Jintao and other national leaders have ordered an investigation into the snowballing reports of widespread use of slave labor. The central government has dispatched an investigative team headed by the Ministry of Labor and Social Security, Xinhua reported late Friday.

The raids were prompted in part by an open letter posted online 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.