Around 80,000 people rallied in Barcelona on Sunday in a show of support for Catalan leaders locked in a tug-of-war with Madrid over independence for their region, police said.
In November 2014, they staged a symbolic vote on independence for the wealthy northeastern region in defiance of Spain's central government.
Mas is now to stand trial on charges of "serious disobedience" and "malfeasance," for which he risks a 10-year ban on holding office.
Catalans have a centuries-old culture with its own language and customs. Demands for autonomy have been fuelled by Spain's economic downturn, leading many to resent sending tax money to Madrid to prop up poorer regions.
On October 20, Spain's Constitutional Court cancelled a ban on bullfighting in Catalonia -- an issue that became emblematic of the tensions.
Carles Puigdemont, the current president of Catalonia, has promised he will call a referendum on whether to secede from Spain in September 2017.