More than 1,000 labourers have been freed in Brazil by the government's anti-slavery team.
Many slaves are used to clear Amazonian land for cultivation
They were said to be working in inhumane conditions on a sugar cane plantation in the Amazon.
An ethanol-producing company which owns the plantation has denied allegations of abusing the workers.
Human rights and labour organisations believe that between 25,000 to 40,000 people could be working in conditions akin to slavery in Brazil.
Many farmers in the Amazon region who incur debts are forced to work virtually for free in order to repay the money they owe.
Labour ministry officials and prosecutors discovered more than 1,100 workers working 13 hours a day and living in conditions described as "appalling".
It is the largest such raid in Brazil, a country beset by the problem of slave labour.
Officials said that the labourers lived in overcrowded conditions with no proper sanitation facilities.
The plantation was located about 250 km (155 miles) from the mouth of the Amazon river near the town of Ulianopolis.
Many workers in the Amazon work on plantations to pay debts
The company which runs the plantation denies the charges against it and said that the workers were paid good wages by Brazilian standards.
But the BBC's Gary Duffy, in Sao Paulo, says many are thought to fall into debt slavery by paying for transportation to work far from where they live and by buying overpriced tools and food.
Ethanol sells in Brazil at half the price for conventional petrol and is said to be a greener fuel for cars.
Recently, President Luiz Inacio Lula da Silva pledged to bring industry leaders and workers together to "to discuss the humanisation of the sugar cane sector in this country".
He was acting after being criticised for calling Brazil's ethanol producers "national and world heroes", despite critics accusing producers of exploiting workers in the sugar cane and ethanol industry.
The Mobile Verification Task Force, which conducted the raid on the plantation, was founded in 1995 by the Labour Ministry and claims to have freed more than 21,000 workers from debt slave conditions at more than 1,600 farms across Brazil.
The Roman Catholic Church estimates there are some 25,000 workers living in slave-like conditions throughout Brazil, most of them in the Amazon.