Kajsa Wahlberg remembers well the reaction when she helped lead efforts to introduce Sweden's now-famous laws criminalising the purchase of sex. "It had enormous interest. People were laughing in 1999 at Sweden and saying it can't be done. A German police officer told me, 'You're crazy sweetie, you can't do that, you cannot prohibit men from buying sex, it's totally impossible.' But he said if you can reduce the amount of trafficking cases with your legislation I wish you good luck, because in Germany it's grown out of proportion."

Nearly 15 years on, Sweden believes it has. Police say the number of prostitutes has dropped by two-thirds. A report by a Swedish academic says that by tackling demand, Sweden provides some of the best protection to trafficking victims.

"There is more that could be done, for example when it comes to implementation there are big gaps so far, but it seems that in Sweden we have much lower levels of trafficking for prostitution than other countries and this is probably one of the main reasons," says Marta C Johansson, author of a five-nation study called Still Neglecting the Demand That Fuels Human Trafficking.

Simon Haggstrom, an officer in the prostitution unit of Stockholm police, is on the frontline of this push to stop men from paying for sex. "My job is to arrest as many men buying sex as possible and I think I have arrested about 700 men since 2007. [They] should know that they are taking a huge risk: they are considering going out into the central parts of Stockholm actually buying another human being. We will go after them."

He says the number of prostitutes has dramatically decreased since the law was introduced, from 2,500 across Sweden in 1998 to about 1,000 today.

Sweden is held up as a model for European reform on prostitution law. Last week, France moved in the same direction, bringing in fines for people who pay for sex. Politicians and police officers from several countries have visited Stockholm, wanting to know what impact the law has had. But the debate is highly polarised. Many experts argue that only by regulating the sex trade and bringing it into the open, as has been done in Germany and the Netherlands, will women get the protection they need.

Linda's story illustrates the complexity of the challenge facing those tackling sexual exploitation. Linda was just 10 when she began a conversation with a 37-year-old Swedish man online. Bullied at school and with her parents divorcing at home, she was an easy victim to his promises of love. Within a few months she considered him her boyfriend and agreed to meet a friend of his in a hotel for dinner. The man took her to her room where he violently raped her. Her boyfriend had sold her, and so strong was his control over her that he was able to do so again and again.

"He guilted me [saying] that if you don't do this next time better or longer, I am going to leave you. This was only three months after our first contact and he had already broken me down so far and so much that I would die for him.''

Over the next five years she was abused and raped by, she believes, about 600 men who were making payments to her boyfriend. "It's almost like a takeaway meal, they are in the hotel room, I come to them, they use me and I leave. The rapes became more and more violent, more and more sadistic. There is a lot of weird porn out there, a lot of very sadistic things. When you watch lots of this kind of porn I can in some way understand that normal sex is not so much fun. This is epidemic and the thing is, we don't really talk about it – not in schools, not at home. I think you need to educate people."

Eventually, Linda's mother discovered what was going on and contacted the police, finally ending the abuse.

Johansson argues that there is not enough protection for young people exploited domestically in Sweden, compared with the UK or the Netherlands, where all minors forced into the sex trade are seen as victims of trafficking. But she believes that criminalising all buyers is the most effective way to stop men knowingly or unknowingly buying sex from a trafficked woman.

"It's important Europe focuses on the issue of demand as it is what fuels human trafficking by making it profitable. It is insufficient to focus only on the traffickers while ignoring those paying for the services of victims – the market must be tackled."

One of the main arguments of those who oppose any attempt to criminalise prostitution is that it simply drives the industry underground, putting the sex workers in a more vulnerable position.

Pye Jakobsson a spokeswoman for the Rose Alliance, representing Swedish sex workers, says: "You can't talk about protecting sex workers as well as saying the law is good, because it's driving prostitution and trafficking underground, which reduces social services' access to victims."

She doesn't believe police figures suggesting prostitution has decreased, saying the numbers represent those women selling sex on the street, not the 50% who work indoors. And she is dismissive of Sweden's pioneering role in European prostitution reform. This law, she says, "is about Sweden selling its ego, showing how brilliant and smart they are, a perfect democracy and country in Europe that others should aspire to".

But in her office, where she surveys the constant flow of evidence of sexual exploitation, Wahlberg believes that the Swedish way is the best chance of helping the most vulnerable women.

"We have a small group of pro-prostitution lobbyists that are very powerful. The sex purchase act was not passed for them; it was passed for the majority of women who suffer from prostitution. If women want to be in prostitution and don't want any help, we don't interfere. But it bothers me that they make themselves spokespersons for these women we are trying to protect because they don't have a voice, where is their voice?"