MONTEVIDEO, Uruguay — General Gregorio Alvarez, the last leader of Uruguay’s brutal dictatorship of the 1970s and 80s died Wednesday at 91 while serving a sentence for human rights abuses.

The military’s health service said he died at the Central Hospital of the Armed Forces in Uruguay’s capital.

Under General Alvarez, Uruguay was part of the secret alliance of South American dictatorships known as ‘‘Operation Condor,’’ in which the military leaders cooperated in persecuting and killing one another’s dissidents.

General Alvarez, himself the son of a general, participated in the 1973 coup that dissolved Congress after a government crackdown on the Marxist Tupamaro rebels who were trying to seize power by force. One of the dictator’s brothers was killed by Tupamaro attackers in 1972.

General Alvarez became army chief in 1978, finally taking over the presidency in 1981. His government imprisoned democratic critics and censored the press.

General Alvarez finally agreed to hand over power to an elected civilian government in 1984 as the wave of dictatorships in the region was starting to recede. Argentina’s military rule ended a year earlier and Brazil’s a year later.

Hundreds of suspected leftists were arrested and tortured during the dictatorship and historians say an estimated 180 Uruguayans were killed — most of them while in the custody of Argentine allies.

In 2009, General Alvarez was sentenced to 25 years in prison for his responsibility in the death or disappearance of 37 Uruguayans under Operation Condor.