India should take action against state-backed vigilantes active in the central state of Chhattisgarh, US-based Human Rights Watch (HRW) says.
Since 2005, security forces and members of the Salwa Judum militia group have killed and raped villagers, HRW says.
Salwa Judum was launched in 2005 to fight the Maoist rebels in the area.
HRW also says the Maoists have kidnapped and executed civilians and targeted people suspected of supporting Salwa Judum.
About 6,000 people have been killed in violence linked to the Maoist rebels in India over the past 20 years.
Chhattisgarh officials deny supporting Salwa Judum and describe it as a "spontaneous citizen's anti-Maoist movement".
"Human Rights Watch has found that since mid-2005 government security forces and members of the Salwa Judum attacked villages, killed and raped villagers, and burned down huts to force people into government camps," a new report released by HRW in Raipur, capital of Chhattisgarh state, says.
The group says it has collected more than 50 eyewitness accounts of attacks involving government security forces in 18 different villages in Dantewada and Bijapur districts in Chhattisgarh.
"Judum and police came to our village... They beat the village official and the priest. They beat others also," the report quotes a villager who fled his village in Dantewada district as saying.
"The people who came to our village had bows and arrows, sticks, and the police had rifles. From our village they also raped a 20-year-old woman. They raped her and left her in the village itself," he said.
"At the same time," the report says, "the Naxalites (Maoists) have carried out bombings, and have abducted, beaten, and executed civilians, particularly those suspected of supporting the Salwa Judum."
Human Rights Watch called on the Naxalites "to immediately end all attacks against civilians and allow camp residents to return to their home villages".
The report says the violence has displaced tens of thousands of people who are stranded in government camps in Chhattisgarh or in the forestlands of neighbouring Andhra Pradesh state.
"The Chhattisgarh government denies supporting Salwa Judum, but dozens of eyewitnesses have described police participating in violent Salwa Judum raids on villages - killing, looting, and burning their hamlets," the report quotes Jo Becker, a member of the Human Rights Watch research team, as saying.
The report - titled "Being neutral is our biggest crime" - is based on four weeks of ground research in Chhattisgarh and Andhra Pradesh in late 2007 and early 2008.
"The conflict has given rise to one of the largest internal displacement crises in India - at least 100,000 people have resettled in camps in southern Chhattisgarh or fled to neighbouring states, principally Andhra Pradesh," according to the report.
"Thousands of families have lost their land, homes, and livelihoods, and now survive in crowded and decrepit camps with little assistance," it says.
"Chhattisgarh officials should help restore the lives of those who wish to return to their homes, and improve conditions for those who fear returning."
The report says both the Maoists and the police have also recruited and used children in the conflict.
"While the Chhattisgarh police have acknowledged this as an error, the government is yet to devise a scheme for... rehabilitating them," the report says.
Chhattisgarh police chief Vishwa Ranjan said the government would respond to the report after studying it.
Maoist fighters, who are waging a violent battle in almost half of India's 29 states, have been described by Prime Minister Manmohan Singh as the single biggest threat to India's security.
The rebels are active in states across east and central India.
They focus on areas where people are poor but there is great mineral wealth.
The rebels say they represent the rights of landless farmhands and tribal communities.