Accused by prosecutors of sedition and rebellion over the region's independence drive, which carry sentences of up to 15 and 30 years in jail respectively, the six had been summoned to court for questioning by a judge.
It was as yet unknown why their hearings were adjourned by the court, which deals with cases involving lawmakers.
Nearby in the Spanish capital's National Court, which hears major criminal cases, eight former Catalan government members were being questioned after also being summoned in the investigation into the secession drive.
They decided to stay put in Belgium, slamming in a statement a "political trial dictated by the Spanish government.