Madrid issued a European arrest warrant for Puigdemont and four of his former ministers after they fled to Brussels last month and ignored a summons to appear before a Spanish judge, claiming they would not get a fair trial.
A judge in Brussels will hear arguments behind closed doors from prosecutors and lawyers for the Catalan separatists over Spain's extradition request in the first round of what could become a protracted courtroom battle, with both sides expected to appeal if they lose.
"We are going to ask the Belgian judge to respect fundamental EU rights," Michele Hirsch, a lawyer for two of the ex-ministers, told AFP.
"The act of organising a referendum is not a matter for criminal law. It is clearly a political opinion that is being targeted, and the peaceful and democratic execution of a series of events linked to that opinion," she said.
More From This Section
An aide to Puigdemont told journalists he had arrived at the Palace of Justice in Brussels shortly before the scheduled 2:00 pm (1300 GMT) start of the hearing.
Belgian Prime Minister Charles Michel met his Spanish counterpart Mariano Rajoy on the sidelines of an EU summit today in Gothenburg, Sweden, though sources said they talked about "the future of the EU" and other issues -- not Catalonia.
Yesterday night Puigdemont had dinner at the home of a politician from the Flemish separatist N-VA party.
The hearing is the latest act in Spain's biggest political crisis in decades, sparked by a banned October 1 referendum that the Catalan parliament then used as a mandate to declare independence.
The five who fled to Brussels went to a Belgian police station on November 5 to answer the warrant and were freed that evening after hours of questioning, on condition that they not leave Belgium.
Under Belgian law a decision on a European arrest warrant -- brought in by the EU to speed up the once-lengthy extradition process in the bloc -- should be made within 60 days.
The case will be heard by a Dutch-speaking judge -- defendants in linguistically divided Belgium have the option to choose which language the judge hearing their case speaks, and the Catalans have won support from Dutch-speaking Flemish separatists in Belgium.
But at the weekend the country's justice minister warned that with appeals, the process could last up to three months.
Puigdemont, 54, who still describes himself as Catalonia's "president", has been active in the media since arriving in Belgium, but his efforts to internationalise the crisis have fallen flat with EU leaders who have closed ranks behind Madrid.
In an interview with Scotland's former independence leader Alex Salmond on the Kremlin-backed RT outlet yesterday, Puigdemont remained defiant, declaring "we will win finally, democracy will prevail".