Prosecutors in Madrid have asked for eight sacked members of Catalonia's regional government to be jailed over their role in October's disputed independence referendum.
Nine Catalan officials testified at Spain's high court over accusations of rebellion, sedition and misuse of public funds.
Ousted Catalan leader Carles Puigdemont and four others disregarded a summons.
The deposed leader, who is in Belgium, said the trial was "political".
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 happening?
Prosecutors asked the high court judge to jail eight of the nine members who turned up for questioning.
Those included sacked deputy leader Oriol Junqueras, Interior Minister Joaquin Forn, foreign affairs chief Raül Romeva and spokesman Jordi Turull.
The ninth, Catalonia's former business minister Santi Vila, should be granted a €50,000 ($58,000; £44,000) bail, prosecutors said. He resigned before the Catalan parliament voted for independence on Friday.
The Catalan leaders are yet to be formally charged. They were accused of rebellion - which carries a maximum 30-year jail term - as well as sedition and misuse of funds.
A judge will 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 did not show up?
Five sacked officials stayed in Brussels, including Mr Puigdemont, who had previously said he would not return to Spain if he and his colleagues did not receive unspecified guarantees of a fair trial.
Reports suggest some of them requested to appear before the judges via video conference.
Mr Puigdemont's Belgian lawyer told Reuters news agency that he would co-operate with the authorities in Spain and Belgium, but did not appear before the judges because "the climate is not good".
The sacked leader's handling of the crisis has drawn criticism among some other Catalan politicians, with left-wing parliamentary deputy Joan Josep Nuet criticising him for creating "yet more bewilderment".
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.