The Ethiopian government's paramilitary force summarily executed 10 men after a dispute with villagers spiralled out of control, Human Rights Watch has claimed.
The attack took place in Ethiopia's eastern Somali region in March. A team for Human Right Watch made the discovery while on a fact-finding mission in neighbouring Somaliland in April.
The watchdog said the special police unit known as the "Liyu police" committed abuses in a village and killed several residents.
The summary executions took place following clashes between residents of Raqda village, in the Gashaamo district, and the Liyu.
HRW says police killed a villager who was trying to protect residents. His death sparked retaliations in which seven police officers were killed.
The security forces responded by launching an "operation" in which they executed 10 men who were in their custody and killed another nine villagers in gunfights.
Human Rights Watch spoke to 30 relatives and witnesses who fled the violence and crossed the border into Somaliland.
Witnesses said that on 17 March the Liyu arrested 23 men and put them in a truck heading towards a neighbouring village.
After a few minutes, police stopped the vehicle, ordered five randomly selected men to get out and shot them by the roadside.
"Three police shot them," a detainee told Human Rights Watch. "They shot them in the forehead and shoulder - three bullets per person."
The remaining detainees were threatened and driven on to the neighbouring village of Adaada where four men were summarily executed.
Villagers said the unit went on the rampage, shooting in the air and dragging villagers out of their houses. They also looted and ransacked homes and shops, witnesses said.
"The killing of several Liyu police members doesn't justify the force's brutal retaliation against the local population," said Leslie Lefkow, deputy Africa director at Human Rights Watch. "The Liyu police abuses in Somali region show the urgent need for the Ethiopian government to rein in this lawless force."
Ethiopian authorities created the Liyu ("special" in Amharic) police in the Somali region in 2007 following an armed conflict escalated between the insurgent Ogaden National Liberation Front (ONLF) and the government.
The special unit has become a prominent counter-insurgency force but has been accused of a series of abuses against civilians across the region.