Medicos del Mundo estimates there are some 350,000 women in prostitution in Spain and 80% per cent of them are foreigners without legal papers.
Spain has introduced a new draft law aimed at cracking down on prostitution.
The proposed legislation will punish those who financially exploit prostitutes, pay for their services, or knowingly provide premises for the practice of prostitution.
Under the law, citizens will be fined if they pay for prostitutes, while pimps or procurers face prison terms of between three and six years. Clients could also face jail sentences if the prostitute is a minor.
But the bill would not make prostitution illegal in Spain, one of the world's leading markets for the practice.
Medicos del Mundo estimates there are some 350,000 women in prostitution in Spain and 80% per cent of them are foreigners without legal papers. Online adverts for prostitutes are also a common sight in Spain.
The group says that many prostitution customers arrive in Spain abroad, either as tourists, business visitors or truck drivers.
The new draft law was admitted to the lower house of Spain's parliament on Tuesday evening, ahead of a vote in the coming months.
“In a democracy, women are not for purchase nor for sale,” said Adriana Lastra, the Socialist party’s deputy secretary-general.
“People who turn to women for prostitution participate directly in the network that shores up this serious violation of human rights,” the party said on its parliamentary Twitter account.
Some political parties and experts have argued that Spain should regulate prostitution to protect sex workers from being further exploited by traffickers.
The Socialists say the bill is aimed at banning pimping in all its forms and offers prostitutes protection as victims of a crime.
Other European countries -- including France, Norway, Sweden, and the United Kingdom -- have introduced similar laws that punish people who pay for prostitutes.