At least 75 people have been killed during weeks of protests in Ethiopia which have seen soldiers and police firing on demonstrators, Human Rights Watch said on Saturday.
"Police and military forces have fired on demonstrations, killing at least 75 protesters and wounding many others, according to activists," HRW said in a statement.
There was no immediate response from Addis Ababa, but it has previously put the toll at five dead.
Government spokesman Getachew Reda said the "peaceful demonstrations" that began last month had escalated into violence, accusing protesters of "terrorising the civilians."
The protests began in November when students opposed government proposals to take over territory in several towns in the Oromia region, sparking fears that Addis Ababa was looking to grab land traditionally occupied by the Oromo people, the country's largest ethnic group.
Demonstrations have taken place in the towns of Haramaya, Jarso, Walliso and Robe among others.
"Human Rights Watch received credible reports that security forces shot dozens of protesters in Shewa and Wollega zones, west of Addis Ababa, in early December," HRW added.
"Several people described seeing security forces in the town of Walliso, 100 kilometres southwest of Addis Ababa, shoot into crowds of protesters in December, leaving bodies lying in the street."
Pictures have appeared on social media, apparently showing bloodied protestors and armed police firing tear gas at student demonstrators.
"The Ethiopian government's response to the Oromia protests has resulted in scores dead and a rapidly rising risk of greater bloodshed," HRW's Leslie Lefkow said.
"The government's labelling of largely peaceful protesters as 'terrorists' and deploying military forces is a very dangerous escalation of this volatile situation."
Rights groups have repeatedly criticised Ethiopia's use of anti-terrorism legislation to stifle peaceful dissent.
With nearly 27 million people, Oromia is the most populous of the country's federal states and has its own language, Oromo, distinct from Ethiopia's official Amharic language.