PHNOM PENH, Cambodia — At night in Cambodia's capital, parks once populated by sex workers fell silent. Streets and abandoned lots in the center of Phnom Penh where drug addicts and homeless slept lay empty. The city's underbelly had been washed away.

Then reports of abuse emerged. Sex workers said police had detained them for weeks, taking the cash they had on hand and raping them — even those who protested by saying they had HIV. There were accounts of government facilities where drug users, street kids and the mentally ill were beaten and starved. Rights workers reported a security crisis for the groups they served, and a facility was shut down after they and the U.N. raised concerns.

That was more than a year ago and the uproar has since eased. Now, a new report has put the government's street sweep campaign front and center again.

In a report released Jan. 25, Human Rights Watch describes a climate of "sadistic violence" in the government's drug rehabilitation centers. Drug users face beatings and arduous forced labor, while being deprived of effective treatment for their addiction, the watchdog group says.

"He had three kinds of cable … he would ask you which one you prefer. On each whip the skin would come off and stick to the cable," the report quotes a 16-year-old identified as M'noh as saying.

In its own study, the World Health Organization found a nearly 100 percent relapse rate in people coming out of the government's drug rehabilitation facilities. "This is a common approach globally," says Graham Shaw, a technical adviser for the World Health Organization in Cambodia. "It's cheap and easy and it allows the government to show the public that it's responding to drug dependence problems amongst the population, but it doesn't provide a solution."

The facilities are presided over by a mix of authorities, including local government offices, the Social Affairs Ministry as well as civilian and military police. Human Rights Watch says officials running the rehabilitation centers profited by renting out detainees as laborers and by selling blood they forced detainees to donate. More than 2,000 people were detained in 11 of these facilities throughout the country in 2008, the vast majority involuntarily, according to the group.

"The real motivations for Cambodia's drug detention centers appear to be a combination of social control, punishment for perceived moral failure of drug use and profit," says the report.

The report sheds light on the government's controversial use of holding centers for drug users, homeless people, sex workers and beggars — who are often rounded up before national holidays and visits by foreign dignitaries, when the capital is on display. Rights groups have long called for the closure of such facilities, citing frequent allegations of violence and forced detention, and questioned the effectiveness of the treatment programs they supposedly offer.

The issue came into the spotlight in 2008, after the government launched a contentious law that outlawed prostitution. In the months following the law's implementation, police carried out a series of raids on brothels and street-based prostitution that rights groups said gave police free rein to rape and rob sex workers they detained. They say the law has done little more than drive prostitution deeper underground, making sex workers more vulnerable to trafficking and pushing them further away from the public health groups that have been instrumental in curbing the country's HIV/AIDS rates.

"This sort of 'cleaning' the streets of undesirable people has been happening for a long time, but there's been more attention towards it recently," said Mathieu Pellerin, who works with the local rights group Licadho.

According to Pellerin, when a Licadho outreach team was able to gain access to one of the government's main poorhouses in 2008, they found an elderly women in her dying moments being left untreated and a young mother nine months pregnant who would have given birth in her cell without any assistance had they not been able to convince the facility to release her.

The government has denied reports of violence and mistreatment in its facilities. "There's no violence, rape, nothing like that" in the drug centers, said Neak Yuthea, who is head of the government's rehabilitation program. "Drug addiction is a new problem in Cambodia. This is good for them. … Maybe Human Rights Watch wants to see the drug users living on the street."

The government says most drug users are interned at the request of their families and that many homeless volunteer to live temporarily in the centers because they are given food, a roof over their heads, and, in some cases, basic vocational training. It has also cited a lack of resources in some cases to explain substandard facilities.

Licadho's director, Naly Pilorge, says, however, it isn't simply a matter of underperforming social welfare. "You have people who have done nothing wrong who are detained like criminals," she said. "It could be a construction worker who doesn't look like he belongs on the street where he is … or a poor-looking kid who is just walking along the street."

Skyscrapers and condos are fast rising in Phnom Penh but rights groups here say real development will remain illusory as long as the government sweeps the country's social problems under the rug.