Spain is bracing for the nation's most sensitive trial in four decades of democracy this week, with a dozen Catalan separatists facing charges including rebellion over a failed secession bid in 2017.

The proceedings, which begin Tuesday, will be broadcast live on television and all eyes will be focused on the impartiality of the Spanish Supreme Court.

Catalonia's separatists have attacked the court's credibility in the run-up to the trial, saying it is a puppet of the Spanish government and any ruling will be a political one that has been decided in advance.

"In reality, it's democracy itself that will go on trial," Oriol Junqueras, one of the accused, wrote from jail in reply to questions sent by The Associated Press. "We are before a trial which, through a partial investigation full of falsities and irregularities, criminalizes a political option and an ideology."

But Supreme Court president Carlos Lesmes dismisses that notion, saying the trial is the most important since Spain's transition to democracy in 1977 after the death of dictator Gen. Francisco Franco.

"This is a trial following the highest standards set by the European Union," Lesmes recently told a group of journalists.