Sacked members of Catalonia's regional government are appearing at Spain's high court to face rebellion and sedition charges, after October's disputed independence referendum.
Nine officials have turned up at the court in Madrid for questioning.
But ousted Catalan leader Carles Puigdemont and four others have stayed away. Prosecutors could order their arrest.
Mr Puigdemont, who is now in Belgium, said: "This is a political trial."
Spain has been gripped by a constitutional crisis since the referendum was held on 1 October in defiance of a constitutional court ruling that had declared it illegal.
Last week, Spanish Prime Minister Mariano Rajoy imposed direct rule on Catalonia, dissolving the regional parliament and calling snap local elections.
This came after Catalan lawmakers voted to declare independence of the affluent north-eastern region.
The Catalan government said that of the 43% of potential voters who took part in the referendum, 90% were in favour of independence.
On Monday, Spain's chief prosecutor said the Catalan leaders were accused of rebellion - which carries a maximum 30-year jail term - as well as sedition and misuse of funds.
They were ordered to appear in Madrid's Audiencia National (high court) on Thursday morning for questioning.
They are yet to be formally charged. A judge will have to decide whether the officials should go to jail pending an investigation that could potentially lead to a trial.
The judge can also grant them conditional bail and order them to surrender their passports.