HONG KONG (AFP) - Chinese sex workers are being subjected to widespread abuse by authorities, including beatings and torture in police custody and detention without trial, Human Rights Watch said on Tuesday.

"Sex workers are treated as if they have no rights," the international watchdog's China director Sophie Richardson told a press conference in Hong Kong during the launch of a report on the subject.

"Rather than being protected by police, sex workers are regularly subjected to beatings, ill-treatment and torture in custody," she said, adding they could be detained in "re-education through labour" camps for up to two years without trial.

"I was beaten until I turned black and blue because I wouldn't admit to prostitution," said one woman, named Xiao Yue in was quoted as saying in the report.

Another told how she and two other sex workers had been tied to trees by police, had cold water thrown over them and were then beaten.

The report said some of the abuses suffered by sex workers in custody "constitute torture under domestic law".

It added that the government periodically carries out vigorous nationwide crackdown campaigns against prostitution and pornography, including raids on sex worker venues and the detention of large numbers of women.

Sex workers who have reported crimes against them, including rape, have themselves been arrested after revealing to police what they do for a living, Richardson said, as prostitution of any kind is illegal under Chinese law.

"I've been raped several times," one sex worker identified as Mimi was quoted as saying in the report.

"But because I am a sex worker, and selling sex is a violation of the law, I could be arrested. So I have never been willing to report to the police. I just have to grin and bear it."

Sex workers are also subjected to forcible testing for HIV, with their privacy and patient confidentiality disclosed to third parties, the report said.

Richardson said there had been "an extraordinary boom in sex work" since Chinese economic reform began in the late 1970s and there were now an estimated four to six million sex workers in China.

They are considered a "social evil" and are often referred to by Chinese officials as fallen women, she said.

The report found that poverty was one of the driving factors leading women to become sex workers, alongside a lack of education and employment opportunities, job loss, divorce or separation.

In China's poorest regions gender inequality is endemic, with women twice as likely as men to be illiterate, the report said.

It added that many women over the age of 40 had turned to prostitution after being laid off by state enterprises in the late 1990s.

The New York-based watchdog conducted studies between 2008 and 2012, mainly in the Chinese capital Beijing, including interviews with 75 sex workers.

Zi Teng, a Hong Kong-based group that offers support to hundreds of migrant sex workers from mainland China in the city said it hoped the report would raise international awareness of the issue.

Spokeswoman Ann Lee also called for the Chinese government to decriminalise sex work and make it a legitimate job to prevent future abuses.

"If the sex workers are victimised, or criminals go to their place and harass them, sex workers can then go to the police directly and ask for help," Lee said.

One of the HRW report's key recommendations was the scrapping of criminal and administrative punishments for voluntary consensual sex.

"Respecting consenting adults' autonomy to choose to engage in voluntary sex work is consistent with respect for their human rights," it said, adding that criminalisation prevented sex workers from accessing justice and government protection from violence.