A French law cracking down on prostitution has led to a surge in police harassment, driving Paris sex workers underground without proving effective against trafficking, a report by rights groups said on Thursday.

According to the results of a year-long inquiry by a committee of rights groups, lawyers and magistrates unions, many prostitutes face daily "police and judicial abuses" following the adoption in 2003 of a law making it a criminal offence for them to solicit for sex. .

"Prostitutes have had more and more trouble with the police, who seem to confuse soliciting and prostitution," said Marie-Agnes Combesque, who took part in the investigation. "Some police officers are behaving like cowboys -- either they are poorly managed, or they are being deliberately allowed to act this way," she said. .

"While prostitutes faced abusive detention periods and racial insults, the report also said it found evidence their clients were "blackmailed" by police to give false testimony to allow sex workers to be convicted for soliciting..

Prostitutes working independently from camper vans in the Bois de Boulogne and Bois de Vincennes -- wooded parks on the Paris outskirts known as hotspots for the sex trade -- were illegally given a string of parking tickets until forced to leave, the report said. "

"Because of police harassment, one self-employed prostitute who has worked in the Bois de Vincennes for 30 years told AFP, the number of sex workers in the area has dropped from 300 to 50.

"The others couldn't take the hassle from the police: they've gone to other woods, further away, and they've fallen into the hands of pimps," she said.

The report also said the law had failed to crack down on proxenetism -- with the number of convictions against stable since 2003 -- or protect the victims of sex trafficking.

"The main police union, Alliance, denied the charges of police abuses -- saying officers were simply applying the law that the number of complaints about prostitution had fallen significantly.

The committee behind the report -- which includes the League of Human Rights, the MRAP anti-racism group, the French lawyers union SAF and the SM magistrate's union -- launched its inquiry after it was approached by a group of several dozen prostitutes complaining of discrimination and abuses.