Thursday's conviction relates to Garzon's decision in 2009 to order wiretaps of jailhouse conversations between lawyers and detainees accused of paying off politicians of the now-ruling conservative Popular Party to obtain lucrative government contracts in the Madrid and Valencia regions.
Such wiretaps are permitted for terrorism cases, but Spanish law is more vague on non-terror cases.
Garzon argued that he ordered the wiretaps because of suspicions the lawyers were being given instructions by the detainees to launder money.
But the Supreme Court said Thursday that Garzon had no legitimate reason to suspect the lawyers, ruling that the wiretaps were not justified and violated the detainees' right to a fair defense.
The judges wrote that Garzon engaged in "practices that these days are only found in totalitarian regimes in which anything is considered fair game in order to obtain information that interests, or supposedly interests, the state."
In the civil war case, prosecutors also say Garzon committed no crime. Those charges stem from a complaint filed by two small right-wing groups.
After Garzon was indicted and suspended in 2010, he took a six-month job in The Hague at the International Criminal Court as an adviser to its chief prosecutor. After that, he accepted a position as a human rights adviser to the government of Colombia, which is fighting leftist rebels and powerful drug lords. So even if Garzon's career ends in Spain, he could presumably accept a similar position.
Protesters said Garzon's conviction was a major setback for Spanish democracy.
"Spanish justice is shameful," said Diego Torrel, a 53-year-old railway construction worker. "It seems like we still have the same judges we had during the Franco era."
Alan Clendenning and Jorge Sainz contributed from Madrid.
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