Spain's chief prosecutor has called for charges including rebellion - which carries a maximum 30 year jail term - to be brought against Catalan leaders.
José Manuel Maza said they should also face sedition charges following the region's declaration of independence.
It comes as Spain acts to take direct control of Catalonia, replacing senior officials.
Meanwhile, former Catalan leader Carles Puigdemont is reported to be in the Belgian capital, Brussels.
Mr Puigdemont is with other dismissed Catalan ministers and will meet Flemish politicians, Spanish media report. Belgium's state broadcaster also reported that he would meet lawyers in Brussels, and a journalist tweeted (in Catalan) that the former president was in a "safe and secret" location.
Spanish authorities sacked Mr Puigdemont as regional leader on Friday, and suspended Catalan autonomy.
Speaking at a press conference earlier, Mr Maza, the Spanish attorney-general, called for Catalonia's leaders to be charged with misuse of funds over the independence referendum they held in October, after it had been declared illegal by the constitutional court.
Under the Spanish legal system, Mr Maza's requests will be considered by a judge.
There appeared to be no major disruption in Catalan government offices on Monday morning, despite some officials defying instructions from Madrid not to turn up for work.
Any ministers who arrived at their offices were given hours to leave under threat of "action" by Catalonia's regional police force, Mossos.
Madrid's temporary move to impose direct control by invoking Article 155 of the constitution - a first for Spain - will see as many as 150 of the region's top officials replaced.
Mr Puigdemont and his vice-president Oriol Junqueras reject the central's government's moves, arguing that they can only be removed from office by the citizens of Catalonia.
Madrid has called for fresh regional elections on 21 December.
A spokeswoman for Mr Puigdemont's PDeCAT party said it would field candidates "with conviction". The ex-president could run in new elections if he has not been jailed by then, according to Spain's Foreign Minister Alfonso Dastis.
On Monday, Mr Dastis said he hoped the forthcoming elections would help to "restore legal governance and rule of law in Catalonia".
Meanwhile, Interior Minister Juan Ignacio Zoido has written to all police officers in Catalonia asking for their loyalty as a "new era" begins in the region
Spain has been gripped by a constitutional crisis since a referendum, organised by Mr Puigdemont's separatist government, was held earlier this month in defiance of a ruling by the constitutional court which had declared it illegal.
The Catalan government said that of the 43% of potential voters who took part, 90% were in favour of independence.
On Friday the regional parliament declared independence.
Spanish Prime Minister Mariano Rajoy then announced the dissolution of the regional parliament and the removal of Mr Puigdemont as Catalan leader.
Mr Puigdemont has urged "democratic opposition" to direct rule from Madrid.
Before this, the region had one of the greatest levels of self-government in Spain.
It has its own parliament, police force and public broadcaster, as well as a government and president.
Catalans had a range of powers in many policy areas from culture and environment to communications, transportation, commerce and public safety.