The ousted Catalan leader, Carles Puigdemont, has called for separatist parties across the political spectrum in Catalonia to form a united front in December’s regional elections, effectively turning the polls into a de facto referendum on his drive for independence.

Tweeting from self-imposed exile in Brussels, the deposed regional president on Saturday launched an online petition yesterday to establish a broad secessionist coalition, the day after Spain issued an international warrant for his arrest.

“It is the moment for all democrats to unite. For Catalonia, for the freedom of political prisoners, and for the republic,” he said, adding that the elections offered voters a decisive choice “between democracy and imposition”. By midday there were more than 30,000 signatures.

A fractious and ideologically diverse coalition of parties triggered the current crisis when they used a slim majority in the regional parliament to plan an illegal referendum on breaking away from Spain and then unilaterally declaring independence last month. If they can unite again and win back that majority in next month’s vote, it would be a huge setback for Madrid’s efforts to defuse the biggest constitutional challenge Spain has faced in nearly four decades.

The central government voted to suspend self-rule and take back control of Catalonia just minutes after the fleeting republic was declared in Barcelona on 27 October. The Spanish prime minister, Mariano Rajoy, then set a new round of regional elections for the earliest legally possible date, 21 December.

It looked like an astute political move, allowing the central government to deflect accusations it was trampling on Catalan democracy, while sowing division among pro-independence parties. With a registration deadline of 7 Novemberthis Tuesday, they were immediately thrust into bitter disputes about whether they should take part in the election or not. Hardliners insisted that running candidates meant implicitly accepting Madrid’s authority and acknowledging that the Catalan republic had been stillborn. Moderates argued that sitting out the election meant losing both political authority and the government funds that are vital to their fight for a separate state.

But now all major parties have said they will take part, and there is a real chance that Rajoy could face another separatist parliament at Christmas. The latest polls suggest pro-independence parties could control the legislature again, if they are once more willing to put their battles with Madrid ahead of all other political priorities. The last separatist coalition brought together groups ranging from Puigdemont’s own centre-right party to the radical left CUP (Popular Unity Candidacy). The central government’s handling of the crisis has angered many in Catalonia, even those who do not support independence, and could bolster support for pro-independence parties.

Carles Puigdemont at a rally celebrating the National Day of Catalonia in Barcelona on 11 September 2017. Photograph: Quique Garcia/EPA

A particular flashpoint has been the arrest and jailing of separatist leaders, including several members of Puigdemont’s cabinet. They could face charges for crimes including rebellion, sedition and embezzlement for their part in pushing for secession. The jailed politicians, who have mostly been denied bail ahead of trial on the grounds they are a flight risk, say their cases are political persecution. The ousted vice-president, Oriol Junqueras, shared a defiant message from his prison cell via Twitter: “We are never going to renounce freedom, even if the price is prison”. A lawyer for the prisoners went public with complaints of “humiliating” treatment, including strip searches.

Their imprisonment has prompted unease abroad, including among allies who have been broadly supportive of the Spanish government and critical of the Catalan independence movement. The Nobel peace prize winner and international negotiator Martti Ahtisaari is among those concerned. He condemned the separatists’ independence bid as a “stupid gamble” that had “caused problems for a lot of others”, but said Spain’s response was too aggressive. “Now there will be an election, and all of the members of Catalonia’s government that now have left the area have to be given the opportunity to take part [in the election] as a candidate,” he told the EU Observer website.

Puigdemont suggested he planned to take part from Brussels, telling a local TV channel: “I can run a campaign from anywhere in the world, as we are in living in a globalised world.” He had already signalled his intention to stay in Brussels as long as possible, retaining a defence lawyer and saying he believed the Spanish courts were politicised, although he has ruled out claiming political asylum.

Puigdemont also made a hollow-sounding promise that he would not seek to cause the Belgian prime minister further problems. “I will be careful about Belgian politics. I am not here to mix with Belgian politics nor to Belgianise Catalan politics,” he said.

The justice minister moved fast to distance the Belgian administration from the Catalan leader’s fate. Unlike other international extradition processes, the European arrest warrant (EAW) lies entirely in the hands of the courts, Koen Geens said. “The executive power does not play any role in the EAW procedure. Everything goes through direct contact between the justice authorities,” the Associated Press quoted him as saying.

Yet for all that the Belgian government has sought to depoliticise the issue, and insist that it is a matter for the independent judiciary, Puigdemont’s arrival has already stirred up domestic political tensions.

The prime minister, Charles Michel, was forced to reprimand his Flemish nationalist minister of migration and asylum, Theo Francken, after he suggested that it was not “unrealistic” to think that Puigdemont could be given political asylum in Belgium. Other MPs attacked him for undermining the country. “Mr Francken created the impression that he was in collusion,” said Olivier Maingain, the president of the francophone liberal party, DéFI, who added: “This will weaken the European role of Belgium.”