Dozens of family members and government officials attended a sombre ceremony at the memorial in Addis Ababa to remember their loved ones, whose bodies were mostly dumped in mass graves.
The museum took three years to complete and honours the dead with photographs of the 1977-78 campaign of state terror carried out under the orders of Mengistu to wipe out his opponents.
“Our aim is to promote unity and tolerance. Ethiopia has had a troubled past, and we don’t want that suffering to be experienced again,” Ayne Tsige, chair of the organising committee, told AFP.
Mengistu, now in exile in Zimbabwe, was sentenced to death on genocide charges two years ago along with 17 of his henchmen following a decade-long trial in Addis Ababa.
The former army lieutenant colonel was a member of the Marxist junta known as the Derg which ruled Ethiopia from 1974 after the ousting of Emperor Haile Selassie until 1991.
Experts say as many as 100 000 people were killed during the campaign as Mengistu sought to transform the country into a Soviet-style workers’ state.