The Brazilian employment ministry said its officials raided 183 farms, the highest number since Swat-style teams were introduced 10 years ago. In total 4,133 workers were freed, with R$7.4m (£1.8m) paid to victims, the ministry said.
"What we know about is the tip of the iceberg," said Father Ricardo Rezende, an anti-slavery campaigner and author of Stepping Out of the Shadow: Slavery for Debt in Contemporary Brazil, the first study of modern-day slavery in Brazil.
Although slavery was officially abolished in Brazil in 1889, Fr Rezende said the official estimate of just 25,000 slaves in Brazil could be way off the mark: "The real figure could be 250,000."
Brazil's modern-day slave trade began to boom again during the 1964 military dictatorship. Following the creation of Sudam, a regional development agency which opened up industry in the Amazon region in 1966, business people and cattle ranchers flocked to the rainforests to make money. The landowners employed middlemen, known as gatos, who found unemployed workers in Brazil's impoverished rural communities, often in north-eastern Minas Gerais.