BRASILIA (REUTERS) - A call home from an anguished daughter in Spain and images from a soap opera about human trafficking for sexual exploitation led a Brazilian mother to realise her daughter was in trouble.

So she called a sexual abuse hotline set up by Brazil's Ministry of Women's Affairs, prompting an international police operation that led authorities to break up a prostitution ring in the Spanish university town of Salamanca last week.

Spanish police raided a nightclub in Salamanca and freed the Brazilian and five other women who were forced to have sex with customers for US$20 (S$24), Brazilian authorities said on Friday.

Tipped off by a call to the Brazilian hotline last June, Spanish police raided a brothel in Ibiza and discovered 28 women of different nationalities who were forced to provide sexual services 24 hours a day. They lived confined to overcrowded rooms and were monitored with television cameras.

The raids highlight the plight of foreign women who are duped by promises of work and better living in Europe only to end up being forced into prostitution to pay off endless debts.

"They are mostly young, pretty and poor women who are promised work in Europe by traffickers who break their dreams," Brazilian Minister of Women's Affairs Eleonora Menicucci said at a news conference.

When they arrive in Europe, their documents are taken away and they are told they have to pay off large debts, she said.

"It is absolutely inhuman what they are doing to our young adolescents," Ms Menicucci said.

The Brazilian government has made the sexual abuse hotline an international service that can be called free of charge from Spain, Portugal and Italy, the countries to which trafficked Brazilian women are mostly taken.

Justice Minister Eduardo Cardozo said human trafficking was a difficult crime to detect because victims were reluctant to tell what had happened to them. He urged them to come forward.

Human trafficking is such an "underground" crime that the government has no data to determine its dimensions, he said.

A soap opera now airing on television in Brazil has brought home the drama of human trafficking for Brazilians. The show, Salve Jorge, is based on the real-life story of a Brazilian woman who was led to Israel and forced into prostitution.