MADRID – Hundreds of thousands of prostitutes working in Spain urgently need labor rights to prevent them from being exploited, one of Spain’s most prominent union leaders said Wednesday.

“This is a strong, powerful sector, and one that is growing,” said Jose Maria Fidalgo, general secretary of Workers’ Commissions, one of Spain’s two major national trade unions.

“In a civilized country, we cannot tolerate a non-regulated sector where hundreds of thousands of people, particularly women and immigrants, are being exploited,” he told journalists ahead of a May 26 conference on rights for sex workers the union is organizing.

Carmen Bravo, the union’s head of women’s issues, said estimates of the number of prostitutes active in Spain ranged from 300,000 to 400,000. More than 90 percent are immigrants and the majority work in roadside brothels.

Police raids on this type of establishment frequently turn up large numbers of eastern European or African women being held in conditions of near slavery, having been brought into the country by traffickers.

“The sector is growing relentlessly,” Bravo said.

Figures on how much the industry is worth are hard to come by, she added. “But we’re talking about astronomical figures.”

One study estimated a national newspaper was making 6 million euros ($7.7 million) a year from selling advertising space for sex services.

Next week’s conference involves political parties, social and legal experts and sex workers themselves. The union hopes it will pave the way for legislation for the sector, which it says is the only way to protect workers.