Indian police have rescued hundreds of child slaves as young as six during days of raids on workshops in the central city of Hyderabad, a senior officer said Friday. 

Police discovered 120 children, some of them sick, underweight and traumatized, during the most recent raids late Thursday on bangle-making and other workshops, as part of a city-wide crackdown on child slavery, the officer said.

"They have chronic skin diseases and were underfed. They are in trauma and visibly shaken," V. Satyanarayana, deputy police commissioner for south Hyderabad, told AFP.

"They were kept in dingy rooms with no ventilation and exposure to harmful gases," the commissioner said.

"The campaign against bonded labour and trafficking will continue," he added.

The children complained of being forced to work 16 hours a day without breaks, and were threatened with violence and no food if they disobeyed orders, the officer said.

Many of the children were transported from the impoverished northern state of Bihar last year after their parents sold them to traffickers for between 5,000 rupees (US$80) and 10,000 rupees, according to rescuers.

Police began a massive clampdown last week against dozens of workshops, tucked away in the city's narrow alleys, after tip-offs from child rights activists and police informers.

Some 220 children were rescued last week when police stormed similar workshops in the city's south, Satyanarayana said. 

Thirty-one traffickers and agents have also been arrested and charged with child slavery, while police are making efforts to reunite children with their families, Satyanarayana said.

TV footage taken after the raids showed cramped and dirty workshops with no windows and discarded children's clothes. Children were seen huddled in a room, looking bewildered and being watched over by police officers. 

Some four million Indian children work as domestic helpers, in roadside restaurants and in factories making clothes and other items, according to government figures released last year. But activists say the actual figures are much higher.

Indian activist Kailash Satyarthi won the Nobel Peace Prize last year for his efforts to halt child labor, including the trafficking of children to be kept as slaves. (+++++)