The Ethiopian government continues to crackdown on demonstrators in Oromiya, Human Rights Watch said on Monday.
Protests began about four months ago and have continued even though the government dropped a scheme which plans to develop an economic zone on what is now farmland around the capital.
Ethiopia's 25-year long development plan is aimed at attracting investment to help industrialise its agrarian economy. Protesters say the scheme would displace many Oromo farmers.
"Security forces, including military personnel, have fatally shot scores of demonstrators. Thousands of people have been arrested and remain in detention without charge," Human Rights Watch said in a report.
"While the frequency of protests appears to have decreased in the last few weeks, the crackdown continues," it added.
However government spokespeople have not yet commented about the accusations.
The government is yet to give a death toll but US based dissidents have said the toll was more than 200.
Detained protesters said to Human Rights Watch that they had been subjected to severe beatings and that they have never appeared before a judge.
According to the report, women suffered sexual assault and mistreatment, while one 18-year old student was "given electric shocks to his feet".
Detention of teachers in order to prevent more protests was reason for some schools and universities to be kept closed, it also reported.
The government has blamed the violence on armed gangs, while opponents have blamed heavy-handed police tactics.
"The Ethiopian government should end the excessive use of force by the security forces, free everyone detained arbitrarily, and conduct an independent investigation into killings and other security force abuses," said Human Rights Watch.