"This is a very important decision as it confirms that amnesty cannot be granted for the crime of genocide," the groups added.
The 89-year-old former general, who ruled the Central American country from 1982 to 1983, is accused of ordering the massacre of more than 1,770 Ixil Maya indigenous people.
He was sentenced to 80 years in prison at an initial trial in 2013 -- the first former head of state in the world to be tried for genocide in a domestic court.
But the Constitutional Court threw out his conviction on procedural grounds and ordered a retrial.
That trial will now begin January 11 but behind closed doors, after the judge in the case agreed with a medical examiner`s report finding Rios Montt has dementia and is no longer mentally fit to appear in court.
If convicted, Rios Montt will not face prison time due to his condition, but would serve his sentence in house arrest or in a medical facility.
Rios Montt and his military intelligence chief, Jose Rodriguez, are charged with orchestrating a scorched-earth campaign in Ixil Maya areas as the government sought to stamp out rural support for leftist guerrilla groups at the height of Guatemala`s 1960 to 1996 civil war.
Rodriguez will be tried separately in open court.
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