Police in China frequently beat, torture and arbitrarily detain suspected sex workers, often with little or no evidence that they engaged in prostitution, Human Rights Watch said in a report Tuesday calling on the government to discipline abusive officers.

Officers sometimes detain women only on the basis of their carrying condoms, thus deterring their use among sex workers and increasing the risk of spreading HIV, the group said.

The government officially views prostitution as an "ugly social phenomenon" and the solicitation, sale and purchase of sex in China are illegal. However, despite frequent government crackdowns, prostitution remains rampant and sexual services are openly offered in massage parlors, karaoke bars and nightclubs.

Human Rights Watch says they interviewed women who told of violence by police and of being detained following sex with undercover police officers. One anonymous woman cited in the report said she and two colleagues were assaulted by police who "attached us to trees, threw freezing cold water on us, and then proceeded to beat us."

Other problems are arbitrary detention of sex workers and discrimination by law enforcement officials when sex workers try to report crimes or abuse, the report said. It focused on women primarily in Beijing who engage in sex work on the streets, in public places such as parks, and in massage parlors and hair salons. While Chinese law treats most sex work-related offences as administrative violations, punishable by fines and short periods of police custody or detention, it allows for administrative detention of up to two years for repeat offenders.

In most of East Asia, prostitution is embedded in a business and political culture of entertaining clients and partners in karaoke bars and nightclubs. Prostitution also is illegal in Japan, but legal gray areas still allow it to flourish. South Korea toughened its anti-prostitution laws in 2004, driving thousands of prostitutes and pimps out of business, although the industry there remains widespread. Still, the level of police abuse against sex workers is deemed lower in those two countries in part because of a stronger rule of law.

"There is a much more robust legal system in both Japan and South Korea so this offers in the first place a greater protection for women who engage in sex work," said Nicholas Bequelin, a senior researcher at Human Rights Watch. "Of course you don't have the kind of limitations on right to expression and right to assembly and so on that you face in China, which is also contributing to this climate enabling these abuses."

Human Rights Watch says that police in China frequently detain sex workers without the evidence required by law that sexual services were provided in exchange for money or property, and that officers have extensive powers to take suspects into custody for periods ranging from several days to several months.

The group said arrests based merely on condom possession boosted the country's risk of spreading HIV. At the same time, the group also condemned forced HIV testing of sex workers by public health agencies and the disclosure of the results to third parties.

Human Rights Watch called on the government to publicly commit to strict nationwide enforcement of provisions that prohibit arbitrary arrests and detentions, police brutality, coerced confessions, and torture, and ensure swift prosecution of abusive police officers. It also called on it to enact legislation to remove criminal and administrative sanctions against voluntary, consensual sex and related offenses, such as solicitation.

"In China, the police often act as if by engaging in sex work, women had forfeited their rights," Sophie Richardson, China director at Human Rights Watch, said in a statement accompanying the report. "The government must abandon its repressive laws against sex workers, discipline abusive police, and end the suppression of sex workers rights advocates."

The rights group interviewed sex workers, clients, police, public health officials, academic specialists and members of international and domestic nongovernmental organizations between 2008 and 2012. It focused on interviews with 75 women sex workers in Beijing, including 20 detailed interviews with women between the ages of 20 and 63.

All the sex workers they spoke with said they had voluntarily chosen sex work. Factors included poverty, job loss, divorce and lack of economic and educational opportunities for women — particularly in the countryside.