Japanese women are being exploited and abused in Japan's multibillion dollar pornographic film industry, rights advocates said on Thursday, calling on authorities to strengthen laws to protect them.

Pornography is widely available in Japan, where some actresses become well known enough to appear on mainstream TV chat shows or as commentators in weekly magazines.

But the dark side of the industry is seldom discussed openly and the rights of those who work in it even less so. In an attempt to shine a light on abusive aspects of the business, a coalition of campaigners and attorneys held a press conference to announce findings of research conducted over a six-month period last year.

More than 130 cases in the past four years have come to their attention in which mostly young women sought help after claiming sexual abuse.

The mistreatment included being forced to engage in repeated intercourse without protection and even being gang-raped during filming. The majority of the cases have their roots in coercive or fraudulent signing of contracts - sometimes targeting minors. Initially, women are casually approached on the street by brokers who pretend to be scouting models, the advocates said.

When victims try to break the contracts they are threatened with lawsuits and so give in.

"These women are forced to perform in such sexually abusive movies against their will," Yukiko Tsunoda, one of the lawyers, said at the press conference.