The Washington Post said the previously unreported killings occurred Nov. 21, 2021, at a camp near the town of Mirab Abaya that housed more than 2,000 detained Tigrayan soldiers.
Witnesses told the Post that between 16 and 18 guards at the camp opened fire on prisoners late that afternoon, prompting many to flee into the bush, chased by Ethiopian soldiers.
The report said after running for an hour, some escapees came across some locals and begged them for help. Instead, a mob of at least 150 people attacked the Tigrayans with machetes, sticks and stones.
Witnesses said members of the mob were incorrectly told the Tigrayans were prisoners of war who were responsible for the deaths of local men in the military.
The Post said none of the soldiers killed had been combatants against Ethiopian forces.
The attacks happened at a time when Tigrayan forces were advancing toward the Ethiopian capital, Addis Ababa. Prisoners interviewed for the story speculated that the attacks were triggered by fear or desire for revenge.
The bodies of the 83 Tigrayans were dumped in a mass grave outside the prison camp, according to witnesses.
"They were stacked on top of each other like wood," said one.
The Post said it interviewed more than two dozen people, including prisoners, medical personnel, officials and local relatives, for the story that was published Monday.
The war between Ethiopia's central government and the northern Tigray region broke out in November 2020. The sides are currently working out details of a cease-fire agreement.