The court on Monday sentenced the former regional vice president, Oriol Junqueras, to 13 years for sedition and misuse of public funds.
The 12 were tried for their actions in a 2017 attempt by Catalonia to break away from Spain following an illegal independence referendum.
Grassroots pro-secession groups have previously said that if any of the defendants were found guilty they would organize protests and "peaceful civil disobedience."
Spanish authorities have deployed hundreds of extra police to the region in anticipation of the ruling.
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