Madrid  - Press photographs of prostitutes plying their trade right out on the street have reignited a debate about the practice in Spain, where it has seen a rapid growth over the past 15 years.

Traditionally perceived as relatively marginal, prostitution is increasingly seen as "normal", with ever younger men preferring to pay for sex rather than taking the trouble of trying to pick up girls.

Prostitution is again making headlines after the daily newspaper El Pais published pictures of prostitutes and their clients having sex on the street at night in a Barcelona tourist neighbourhood.

Barcelona had become increasingly lax in applying a 2006 municipal ordinance which stipulated fines of up to 750 euros (1,050 dollars) for sex workers or their clients, critics complain. The ordinance was aimed at eradicating such practices from central districts.

Associations representing local residents or the prostitutes themselves urged a legalization of the trade, describing it as the only way to guarantee prostitutes adequate working conditions.

The city, however, only deployed more police to chase the sex workers off the streets in the Raval neighbourhood.

The debate revealed a problem affecting, not just Barcelona, but all of Spain, where - like in many other European countries - the authorities have been unable to choose between a Swedish-style ban and a Dutch-style legalization of prostitution.

The practice has thus been left in a legal limbo which makes it more difficult for police to pursue criminals trafficking in prostitutes, leaving thousands of women at the mercy of pimps coercing them into the trade.

The influx of immigrants has led to the growth of prostitution in Spain, where up to 300,000 women are estimated to be selling sex in flats, hostels, streets, parks or at around 2,500 "clubs" functioning as brothels.

One of the world's most lucrative criminal trades moves an estimated 18 billion euros in Spain annually.

Around 90 per cent of the sex workers are migrants from countries such as Brazil, Colombia, Nigeria, Romania or Russia.

There is abundant judicial evidence that many of the women have been lured to Spain with false promises and forced into prostitution with threats, beatings, rape and even outright torture.

"Sexual exploitation rings are very cruel, the roughest I have seen in my career," police officer Carlos Igual said.

However, prostitution is not technically illegal, and police can only act in cases in which prostitutes report their pimps for coercion. That is something that the women, who may be in the country illegally and may not even speak Spanish, are often afraid of doing.

Yet prostitution has also not been clearly defined as a legal activity, preventing police from inspecting brothels to make sure that sex workers are adequately treated.

The government has adopted a plan to combat coerced prostitution, foreseeing measures such as seizing the property of traffickers and giving women 30 days to decide whether to testify against them.

So far, it is mainly on a local level that authorities have taken sporadic action such as information campaigns, police closing premises or placing video cameras in neighbourhoods to dissuade potential clients of prostitutes.

Nearly 30 per cent of Spanish men admit to having bought sex, according to the National Statistics Institute.

Potential clients often downplay reports that many of the prostitutes are de-facto sex slaves. "They wouldn't do it if they didn't like it," a Madrid taxi driver commented.

The abundant offer of cheap women with a variety of exotic looks has made many young men regard prostitution as a normal leisure activity, with the average age of the clients dropping to around 30years, El Pais quoted psychologists and other researchers as saying.

The growth of prostitution reflects "a culture of immediacy, the will to get casual sex fast and without effort," the daily wrote.

Some experts see the increase of prostitution as a new form of oppression against women, but the issue is made more complicated by the fact that some prostitutes do enter the trade voluntarily.

"I could be cleaning stairs, but now I earn 2,500 euros a month," one sex worker said in the southern town of El Ejido. (dpa)