The parliament’s speaker, Carme Forcadell, and five other members of the governing body face possible charges of rebellion, sedition and embezzlement, punishable by decades in jail.
A Supreme Court judge is set to decide after questioning them if any preventive measures - including possible jail - are to be applied while the investigation continues.
Madrid took the unprecedented step of seizing control of the wealthy region shortly after the October 27 independence declaration, the first time in four decades of democratic rule that one of Spain’s 17 regions has temporarily lost its self-government.
Spain removed the regional government, dissolved the parliament and called a new regional election for December 21.
The independence declaration in the Catalan parliament was boycotted by most opposition legislators but held despite previous court rulings. It was passed by 70 votes to 10 in the 135-seat legislative body.
Former Catalan regional president Carles Puigdemont and four of his dismissed cabinet members fled to Brussels, where they are fighting Spanish arrest and extradition orders.
They are also trying to rally European support for their cause. Although no country has publicly sided with them so far, their presence in the Belgian capital is sowing divisions among politicians.
Eight members of the now-dismissed Catalan cabinet and two activists have already been sent to jail as Spain’s National Court studies possible charges of rebellion and sedition. One further official was released on bail, but remains a suspect.
Forcadell remains the parliament’s president, heading a commission of two dozen legislators during the transitional period to next month’s polls.
The Supreme Court judge delayed the questioning for more than a week after the legislators’ lawyers argued they were not given enough time to prepare the defence.
Catalonia, with 7.5 million people, represents a fifth of Spain’s gross domestic product. Polls show its people roughly evenly divided over independence.
The regional separatist authorities claimed a banned October 1 secession referendum gave them a mandate to declare independence.