By Guy DeLauney
BBC News, Phnom Penh
A human rights organisation has alleged that illegal drug users in Cambodia are being subjected to violence, sexual abuse and forced labour.
Human Rights Watch (HRW) says guards in Cambodia's rehabilitation centres are responsible for "sadistic violence".
It says people in the centres suffer "horrible conditions" and do not get the treatment they need to deal with their drug problems.
The Cambodian authorities have dismissed the claims.
"Illegal, abusive and ineffective" - the HRW report passes a damning verdict on rehabilitation centres across the country.
Interviews with dozens of people produced allegations of guards carrying out beatings, electric shocks and even rape.
One former resident claimed that a detainee who tried to escape was tied to a flagpole in the sun while red ants were poured on to his legs.
Attendance at the centres is supposed to be voluntary.
But human rights organisations have said that many residents have been caught in police clean-up operations targeting homeless people and sex workers as well as drug users.
A government spokesman said Human Rights Watch reports were always critical, while one police official said the allegations were "not true".