Thai troops are engaged in "systematic" torture as they combat an insurgency in the south, Amnesty International says.
Troops were using violence to intimidate individuals and communities into ending support for the insurgents, the rights group said in a report.
A Thai commander acknowledged isolated cases of abuse, but said that torture was neither acceptable nor tolerated.
The insurgency in Thailand's Muslim majority south has killed more than 3,500 people since 2004.
Thailand annexed the three southern provinces - Narathiwat, Yala and Pattani - in 1902. The vast majority of people there are Muslim and speak a Malay dialect.
Insurgents target those perceived as collaborating with the government in bomb blasts, beheadings and shootings; their aim is to force Buddhist residents from the area.
In a 37-page report, Amnesty International accused the Thai military of employing brutal violence in its campaign to end the insurgency.
Citing testimony from victims, it said troops had beaten and kicked detainees, given them electric shocks, near-suffocated them and sexually abused them.
In one case, it said, three rubber farmers were detained. One was beaten until he died, the other two were brutally tortured by soldiers who wanted them to admit to being insurgents.
At least four people had died from torture, Amnesty said. Research showed the torture "cannot be dismissed as the work of a few errant subordinates in isolated circumstances", it added.
Lt Gen Pichet Visaijorn, the army's regional commander, said that torture and human rights violations were not tolerated.
"We have a very clear policy that the use of torture is not acceptable and has never been," the Associated Press news agency quoted him as saying.
"The military operation in the south is done in line with a human rights standard that is accepted internationally."
Thailand's new leader, Prime Minister Abhisit Vejjajiva, has promised to work hard to ease tensions in the south.