One of Spain's largest unions, Comisiones Obreras, has called for labour rights for prostitutes.
Trade unionists and sex workers will attend the conference
It argues that legislation is needed to prevent exploitation and is to hold a conference on labour rights on 26 May.
There are an estimated 300,000 to 400,000 prostitutes in Spain, more than 90% are immigrants and most operate from roadside brothels, said the union.
Many are eastern European or African and work in near slavery, it said. "This cannot be tolerated," it added.
"This is a strong, powerful sector, and one that is growing," said Jose Maria Fidalgo, general secretary of Comisiones Obreras.
Many women are brought in by traffickers and do not even have Spanish citizenship, the union said.
It denounced the "cynicism" of those who try to hide the massive economic impact of the sex industry.
Carmen Bravo, head of the women's issues section, said one newspaper had made 6m euros (£4.1m; $7.6m) from selling advertising space for sex services.
The conference later this month will be attended by representatives of political parties, social and legal experts, sex workers and union representatives from Holland, Argentina and Sweden, as well as from Spain.
The union hopes the conference will lay the groundwork for legislation to protect sex workers' rights.