BUENOS AIRES, Argentina — Emilio Massera, a leader of the military junta of Argentina’s bloody dictatorship and the former head of the country’s most notorious political prison, where an estimated 5,000 people were tortured and killed, died here Monday. He was 85.

His death, at a naval hospital, was caused by a cerebral hemorrhage, an army spokesman said. Massera had been mostly bedridden since 2002, when he had a stroke. He had been treated for a heart condition and dementia for years.

Massera, along with Jorge Videla and Orlando Agosti, formed the military junta that, on March 24, 1976, ousted President Isabel Peron, widow of Juan Domingo Peron, the founder of the country’s populist movement.

Known by some historians as the “grand orator” of the dictatorship for his captivating rhetoric, Massera operated the Naval Mechanics School, a clandestine prison known as ESMA where thousands of political prisoners were tortured during the six years of the regime, which collapsed in 1983.

Massera was “the most sinister character in our history because he was a serial assassin, without exaggeration,” Osvaldo Bayer, an Argentine historian, said in a radio interview. “He took advantage of his uniform for power and money.”

A federal court convicted him in 1985 of crimes against humanity and sentenced him to life in prison. Then, in 1990, President Carlos Menem granted him and other coup leaders amnesty in what Menem called a gesture of reconciliation.

Born in Buenos Aires in 1925, Massera was educated at Argentina’s elite naval academy and became a midshipman in 1946. He rose to the rank of full admiral and commander in chief under Isabel Peron in 1974.

In that post, he pressed Peron to fight urban guerrilla groups, persuading her to push for laws to beef up security and to stop the insurgents at any cost. He was ruthless in enforcing measures to maintain public order.

Massera ran the military arm of the dictatorship the first two years. As head of the navy, he was also in control of the prison camp at ESMA in Buenos Aires. The admiral visited the camp often, overseeing torture, which included rape. The military tortured most of its prisoners and dumped them, dead or alive, into the River Plate from airplanes.

Massera studied philology as a young man and obsessed over the use of language. “He had a messianic oratory, a sonorous oratory,” said Marguerite Feitlowitz, a literature professor at Bennington College who has written a book about Argentina’s dictatorship. “His speeches were very evil and very polished.”

In a speech in 1972 to officers who had been chosen to carry out the death flights, Massera said, “Death will not triumph here, because all of our dead, each and every one, died for the triumph of life.”

After Massera was convicted of dozens of counts of murder, torture and robbery, he was unrepentant. “Nobody has to defend himself for having won a fair war,” Massera said after the trial. “The war against subversive terrorism was fair.”

Despite Menem’s pardon, Massera was arrested in 1998 on charges of having stolen babies born to political prisoners, a crime not covered by the amnesty. His ill health kept him out of prison.

In 2005, the Supreme Court, under the administration of President Nestor Kirchner (who died on Oct. 27), declared the amnesties unconstitutional. But the court, confirming Massera’s dementia, suspended all cases against him, as well as extradition requests from Germany, Spain, Italy and France. Those countries had sought him for the killings of their citizens during the dictatorship.