Police reported on Tuesday that they had arrested the leader of a human trafficking network that had been exploiting the women between the ages of 19 and 33.
The group had found the women from Paraguay and the Dominican Republic through family or neighbourhood contacts, offering "decent jobs," according to police.
Instead, the group kept them secluded from the outside world, locking them all inside the same room where they were forced to prostitute themselves 24 hours a day and, if they could find rest, sleep on mattresses thrown on the floor.
"The women were deprived of freedom and shared the dwelling space with their captors, who in turn kept watch so they couldn't escape, even using video surveillance systems at all times," the national police said in a statement.
"They were stripped of their cell phones and documentation, thus remaining cut off from the outside world."
Spanish police received a tip in February from a relative of one of the women from Paraguay that she was being held and sexually exploited against her will due to an outstanding debt.
Investigators then found that the leader of the network had advertised the women and his property online with pictures of them completely naked and allowed customers to share their experiences with the victims through the website's forum.
Police raided the property where the women were kept, as well as another one belonging to the leader, who had been arrested last year for similar offenses.
Prostitution has been decriminalized in Spain for more than 20 years, though pimping and exploitation is illegal. According to international sex worker NGO TAMPEP, as high as 90 percent of prostitutes in Spain are immigrants, mainly from Latin America.
Authorities at the weekend also busted a human trafficking ring and freed 29 Chinese women who were forced to become sex slaves at various nightclubs around the country.
The National Police ask that anyone with tips about possible human trafficking call 900 10 50 90, or email firstname.lastname@example.org.