Ethiopian security forces shot dead peaceful protesters in student demonstrations last week, Human Rights Watch said Tuesday, quoting "credible" claims of a death toll "much higher" than the official figure of eight.
"Witnesses said security forces fired live ammunition at peaceful protesters in Ambo on April 30," HRW said in a statement accusing security forces of using massive force.
The state news agency said last week that mass protests caused "loss of lives and property" in several university towns in Oromia, Ethiopia's largest region.
The government said eight were killed in the violence, which it blamed on "anti-peace" forces, but according to HRW "various credible local sources put the death toll much higher."
"Security forces have responded by shooting at and beating peaceful protesters in Ambo, Nekemte, Jimma, and other towns with unconfirmed reports from witnesses of dozens of casualties," the HRW statement added.
"Ethiopian security forces should cease using excessive force against students peacefully protesting... the authorities should immediately release students and others arbitrarily arrested during the protests," it added.
With nearly 27 million people, Oromia is the most populated of the country's federal states and has its own language, Oromo, distinct from Ethiopia's official Amharic language.
According to local media reports, the students were protesting government proposals to extend its administrative control to several towns in Oromia, sparking fears of land grabs.
But the government accused protest leaders of trying to destabilise the country.
"The forces behind the chaos... have a past violent history," the government statement read, claiming the protests had been encouraged by "media inside and outside the country" for "their evil purpose", without giving further details.
Government officials were not immediately available to comment on HRW's claims, but have routinely dismissed its reports in the past.
Last month government spokesman Getachew Reda told AFP that "we don't take orders" from the group, after HRW criticised the arrest of nine journalists and bloggers.
Addis Ababa said the nine were arrested for "serious criminal activities".