The 18 defendants, among them former Catalan vice president Oriol Junqueras, did not appear at the Madrid court for the hearing, which will focus on technical issues.
A seven-judge panel is hearing from defence lawyers who want the case to be tried by a court in Catalonia instead of the Supreme Court.
The judges were expected to reject these arguments and to decide within a week.
The sensitive trial is likely to start at the end of January or early February — more than a year after Catalan leaders in the northeastern region attempted to break away from Spain in October 2017 by staging a referendum despite a court ban.
They subsequently proclaimed independence but Spain's then conservative government moved swiftly to depose the Catalan executive, dissolve the regional parliament and call snap local elections in December.
Nine defendants are in pre-rial custody, including four who are three weeks into a hunger strike against what they see as their unfair treatment by the justice system.
Prosecutors are seeking jail terms of up to 25 years at the trial that is set to last several months.
It does not concern seven other Catalan leaders who fled abroad after the independence bid, such as deposed regional president Carles Puigdemont, since Spain does not allow for trials in absentia.
Catalan separatist parties consider the defendants to be “political prisoners” and complain that their continued detention constitutes a violation of their human rights.