(Adds arrival and court appearance in Argentina, quote, background; changes dateline from MADRID)

BUENOS AIRES, March 31 (Reuters) - A former Argentine naval officer was extradited by Spain to his home country on Monday to face charges of crimes against humanity during a "dirty war" against suspected leftists.

Ricardo Miguel Cavallo, once code-named "Serpico," was an official at the notorious Naval Mechanics School political prison during a 1976-83 military dictatorship, and is accused of kidnapping suspects who never appeared again.

He had been held in a Spanish jail since 2003, when he was extradited from Mexico to face charges of genocide and terrorism during the dictatorship in Argentina.

On arriving in Argentina on Monday, Cavallo was driven under heavy police escort to a Buenos Aires court where he appeared before a judge to hear some of the charges against him.

He declined to declare at the hearing, and the judge ordered him imprisoned pending trial, a court source said.

"Now we must proceed with maximum speed to make sure that a repressor, who committed aberrant crimes, is not allowed to go free," said Rodolfo Yanzon, a human rights lawyer involved with the case against Cavallo.

Spain's cabinet ruled on Friday that Cavallo could be sent back to Argentina under a treaty between the two countries after several years of legal argument over where his trial should take place.

Spanish prosecutors had requested several life sentences for Cavallo for his part in the torture and deaths of Spanish citizens.

An official report estimates that some 11,000 people were kidnapped and killed during a crackdown on leftists and dissidents during the dictatorship. Human rights groups say the number is closer to 30,000.

The whereabouts and burial places of all but a few are unknown.

When Cavallo was extradited by Mexico to Spain, it was the first time one country had handed over a suspect for trial in another for human rights crimes alleged to have been committed in a third. (Reporting by Feliciano Tisera and Martin Roberts in Madrid and Cesar Illiano in Buenos Aires; Editing by Kieran Murray)

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