Ethiopian security forces have killed more than 400 people in a crackdown on protests in the Oromia region since November, Human Rights Watch said.
Tens of thousands of others have been arrested, the rights group said in a new report, citing the accounts of 125 protesters, witnesses and others.
The student-led protests were sparked by a government plan to expand the municipal boundaries of the capital, Addis Ababa, into Oromia, where farmers fear they will lose their land.
The development proposal has since been retracted.
The security forces used live ammunition for crowd control repeatedly, killing one or more protesters at many of the hundreds of protests over several months, according to Human Rights Watch.
Many of those killed were students, including children under 18, the group said.
Leslie Lefkow, deputy Africa director at Human Rights Watch, said: "Ethiopian security forces have fired on and killed hundreds of students, farmers and other peaceful protesters with blatant disregard for human life.
"The government should immediately free those wrongfully detained, support a credible, independent investigation and hold security force members accountable for abuses."
There was no immediate comment from the Ethiopian government.
Ethiopia's government frequently faces allegations of rights abuses against opposition groups, journalists and minority groups.