Italian police say they have freed at least 100 Poles "kept in a state of slavery" in forced labour camps in the south of the country.
Some 20 people were also held in a joint operation with Polish police targeting a criminal ring that supplied farm workers in Italy's Puglia region.
Officials said the workers were kept in barracks with horrible sanitary conditions against their will.
Those who tried to escape were reportedly raped and tortured.
"To call the situation revealed by the carabinieri [police] investigation simply inhuman does in no way do it justice," Italian prosecutor Piero Grasso told reporters.
"We are talking about conditions similar to those of concentration camps where people were not only exploited for their work but also kept in a state of slavery," he added.
Officials said they were investigating at least four cases of apparent suicide in the camps.
The trafficking ring recruited Poles for seasonal work on Italian farms through newspaper adverts, promising them well-paid and safe jobs, Polish police said.
"Those who applied were charged 400-800 [Polish] zloty (£68-£136; 100-200 euros), plus another 150 euros (£103) when they reached Italy," Polish police chief Marek Bienkowski told a news conference in Warsaw.
Once in Italy, the workers were kept in barracks without heating and light and were watched by armed guards, officials said.
They had been forced to work for up to 15 hours a day, earning between two and five euros (£1.40-£3.40) an hour.
They had to pay for their accommodation and food, which pushed most of them into debt.
At least 20 members of the suspected ring were arrested during the joint operation codenamed Promised Land, police said.