MONTEVIDEO Uruguay's Supreme Court said on Friday that a law allowing fresh investigations of dictatorship-era human rights crimes violates the constitution, a ruling that puts dozens of cases into doubt.
About 200 Uruguayans were kidnapped and killed during the 1973-1985 dictatorship, and the small South American nation remains divided over how to deal with former military officers accused of rights abuses.
Congress passed legislation in 2011 that made new rights trials possible in spite of a 1986 amnesty law shielding most officers from prosecution.
It contradicted the amnesty law by saying rights crimes cannot be subject to a statute of limitations.
Friday's Supreme Court ruling said two articles of the 2011 law were unconstitutional, a decision that effectively re-establishes the amnesty. It drew swift criticism from human rights activists.
"What the Supreme Court has done is protect state-sponsored terrorists," said Irma Leites from the Memory and Justice Assembly group.
The Communist Party said the court was "responsible for defending the impunity of the worst criminals in the nation's history."
Despite the amnesty law, dozens of dictatorship officials have been jailed for rights abuses committed during the dictatorship, including late former President Juan Maria Bordaberry and Gregorio Alvarez, who headed the military government from 1981 and 1985.
Friday's ruling was issued in one particular case, but a magistrate linked to the court called it "a clear signal from the upper echelons of the justice system," suggesting it will serve as a precedent.
Jorge Puente, head of the Centre for Retired Armed Forces Officials, gave a cautious welcome to the verdict.
"We're not satisfied, but we're reassured that things seem to be getting back on track," he said.
Uruguay's moderate leftist president, Jose Mujica, spent more than a decade behind bars for his activities as a member of the Tupamaros guerrilla group in the 1960s and 1970s.
He has urged Uruguayans not to dwell on the brutality of military rule, but he backed scrapping the amnesty as part of his 2009 election platform.