By Katie Paul

BUENOS AIRES, Oct 18 (Reuters) - Argentina opened a major human rights trial on Thursday, the first to focus on crimes committed by the military dictatorship at its most notorious political prison during the 1976-1983 "dirty war."

The Naval Mechanics School in Buenos Aires, known as ESMA, was the government's principal clandestine detention center, where thousands of suspected leftists were tortured and killed.

The court will hear oral testimony against ex-naval officer Hector Febres, who faces four counts of torture charges.

Febres, 66, appeared in court and denied the charges.

Dozens of other ESMA trials are expected to follow his trial.

Human rights groups say that out of an estimated 5,000 people held at the site between 1977 and 1981, about 200 prisoners survived and the rest were drugged and dumped into the ocean from airplanes, or executed by firing squad and cremated in a nearby sports field.

The four ex-prisoners scheduled to testify about their experiences say Febres held them in narrow wooden stalls and used metal bed frames to administer electric shocks.

One man, whose wife and baby were also kidnapped, says officers threatened to slam the baby's head against a wall unless he provided them with information on other leftists.

"A criminal that committed such atrocious crimes cannot be allowed one more day of liberty," said Miriam Bregman, the attorney representing some of the prison's survivors, in a television interview, adding she was requesting the maximum sentence of life imprisonment.

"A minimal recompense would be that he spend his last days in jail, and in a nonmilitary jail at that," said Bregman.

Groups of activists and former prisoners gathered outside the court building for the trial's first day said even that did not go far enough.

Activists complained that courts had not combined cases and that 300 ESMA officials remained free.

"It's been four years and they have done nothing (about ESMA)," added Adriana Calvo, another former detainee. "So, today, not only will we accuse him of kidnapping, but, through these four torture cases and other evidence, we will clearly demonstrate that what happened was genocide."

In Argentina, a wave of human rights prosecutions began in 2003 when President Nestor Kirchner overturned an amnesty law that prevented prosecutions of crimes committed by the military government.

In the past year, three ex-officials, including a former police chaplain, have been convicted of crimes committed during the dictatorship.

ESMA, the best-known of hundreds of detention centers from the dictatorship, was recently converted to a human rights memorial.

Human rights groups estimate that 30,000 people were forcefully "disappeared" by death squads. Most of the bodies have never been located.

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