Brazilian authorities rescued more than 4,500 slaves from captivity last year, carrying out a record number of raids on remote ranches and plantations, according to figures released this week by the country's work ministry.

The government said its anti-slavery taskforce, a roaming unit designed to crack down on modern-day slavery, had freed 4,634 workers from slave-like conditions in 2008. The taskforce, which often works with armed members of the federal police, said it had undertaken 133 missions and visited 255 different farms in 2008. The ministry said former slaves had been paid £2.4m in compensation.

Brazil officially abolished slavery in 1888 but activists believe thousands of impoverished Brazilians are still being lured into debt slavery.

Leonardo Sakamoto, head of a Sao Paulo-based NGO, said slavery remained a big problem despite growing attempts to eradicate it during the government of leftwing president Luiz Inacio "Lula" da Silva.

"It is a very sad situation that leaves you feeling impotent. The federal government has acted - but having slave labour in a country where the wealth is so evident is a very painful contradiction," said Sakamoto, who is a member of Brazil's National Commission for the Eradication of Slave Labour and runs the NGO Repórter Brasil.

Many of Brazil's slave workers come from the impoverished backlands of north-eastern Brazil, where unemployment is high. Rounded up by middlemen who promise them employment, the workers are packed on to coaches and taken to remote farms, often in the Amazon or Brazil's midwest.

Once there, the slaves are put to work producing charcoal, cutting sugar cane or clearing tracts of Amazon rainforest for cattle ranchers. Housed in isolated and often squalid jungle camps, they are forced to work until they have paid off debts for food, medicine and housing. Many lose contact with their families.

Activists claim that ranchers in the Amazon often employ small armies of gunmen to stop workers fleeing.

During a visit to Sao Felix do Xingu, a remote Amazon settlement notorious for illegal deforestation and slave labour, the Guardian met members of the CPT, a Catholic land commission, which often tips off the anti-slavery taskforce.

Maria Nizan de Souza, a CPT representative, said it was common to hear stories of workers being murdered after demanding payment from their employers. She said one rural worker had told her he had seen the body of a colleague floating in a river, bound to a tractor tyre, after he had tried to flee.

Sakamoto said that while the government offered financial benefits to those rescued from slavery, more initiatives to counter poverty and unemployment in the north-east of the country were needed to prevent people from becoming slaves in the first place.