Police on Monday raided an abandoned slaughterhouse in southern Italy that African migrant farmworkers use as a dormitory, part of a recently launched crackdown on labor exploitation.

The raid at dawn near Rosarno, in Calabria, was conducted after migrants — all with residency permits or proper asylum paperwork — told authorities that two fellow Africans forced the 30 residents to pay 30 euros ($33) monthly each to sleep there, some on the floor.

Police also inspected 10 farms and related small businesses, issuing 13 summonses for hiring workers without contracts and other labor law violations. At an orange grove, where most of the fruit-pickers are from the European Union nation of Bulgaria, six women had no contracts at all.

A recent report based on union-compiled data estimates 400,000 farm workers in Italy are exploited. About 80 percent of the poorly paid crop-pickers are foreigners.

Reggio Calabria Prefect Claudio Sammartino told the AP the raids were part of a crackdown launched in November. Earlier raids have seen a total of 13 people put under investigation for alleged exploitation, and, in one case, treating a worker like a slave.

Those who recruit exploited workers for farms risk prison sentences from five to eight years if convicted.

Rosarno is in the heart of the southern farming area that has come under scrutiny for the treatment of farm workers. In 2012, Amnesty International said the migrants were frequently paid less than Italians doing the same job, and sometimes not paid at all. Rosarno is also notorious for tensions between natives and the migrants. In 2010, nearly 40 people were wounded in clashes, and many migrants fled.

"This is a historic wound, and in the past year, we have started an intense effort" to crack down on exploitation, Sammartino said in an interview.

Several dozen farm companies have been inspected, some of which are linked to local clans of the 'ndrangheta, the Calabria-based crime syndicate, said the prefect, who is the interior minister's top official locally.

According to the organization Doctors for Human Rights, most of the Rosarno-area migrants have documents enabling them to stay in Italy, including paperwork proving they are appealing denied asylum requests.

Most of the seasonal workers come from sub-Saharan Africa.

Since formal contracts are required for migrants to receive residence permits, many feel pressured to accept poor conditions in return for legal status for themselves and their families.

Celeste Logiacco, a leader of a farm workers union, estimated that crop-pickers receive about 25 euros a day, or 50 percent less than what's stipulated in the national contract. Logiacco added that Italian workers are also exploited.

"Twenty-five euros a day is not good," Mohamed Ahmed, a migrant from Niger, told The Associated Press recently. While awaiting the outcome of his appeal of a denied asylum request, he lives in a tent camp, with toilets and electricity but no hot water, set up by the Italian government after the 2010 riots.

He says he's in limbo - not earning enough to support his wife and three children who stayed in Niger, but unable to leave Italy since his documents are with Italian authorities while he appeals his case. So Ahmed sells meat and other street food at the camp to make some more money.