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 for 21 December.
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.
What is expected in the Madrid court?
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.
Who has showed up - and who has not?
Mr Puigdemont had previously said he would not return to Spain if he and his colleagues did not receive unspecified guarantees of a fair trial.
His Belgian lawyer told Reuters news agency that he would co-operate with the authorities, but did not appear before the judges because "the climate is not good".
The four other sacked leaders who failed to show up at the high court also stayed in Belgium. Reports suggest some of them requested to appear before the judges via videolink.
Mr Puigdemont's handling of the crisis has drawn criticism among some other Catalan politicians, with left-wing parliamentary deputy Joan Josep Nuet saying his absence could make matters worse for those who followed the court's order.
"The attitude... has been really absurd, managing only to create yet more bewilderment," he told Catalunya Radio.
Among those who showed up were the sacked deputy leader Oriol Junqueras, Interior Minister Joaquin Forn, foreign affairs chief Raül Romeva and spokesman Jordi Turull.
Meanwhile, five other senior members of the Catalan parliament, as well as speaker Carme Forcadell, are facing the same charges but, because of their parliamentary immunity, their cases are being handled by the Supreme Court.
Their hearings have been postponed until 9 November.
Despite Mr Puigdemont's position, the Spanish government insists it has no influence over the country's judiciary, the BBC's Tom Burridge in Barcelona says.
But if those Catalan politicians appearing in court are denied bail it will cause further anger among those who want Catalonia to break away, our correspondent adds.
The court summons also gave them three days to pay a deposit of €6.2m ($7.2m) to cover potential liabilities.
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