Spain's public prosecutor on Wednesday ordered a criminal probe of over 700 Catalan mayors who are cooperating with an October 1 independence referendum deemed illegal by Madrid.
The move increases the pressure on Catalan officials just one day before the pro-separatist camp officially kicks off its referendum campaign in the Mediterranean port of Tarragona.
The prosecutors' office ordered the mayors who have agreed to help stage the vote be summoned to court as official suspects and if they do not appear to "order their arrest", according to a copy of the ruling obtained by AFP.
Catalonia's pro-separatist government has asked the wealthy northeastern region's 948 mayors to provide facilities for polling stations for the plebiscite.
So far 712 mostly smaller municipalities have agreed to participate, according to a list posted on the website of Catalonia's Municipal Association for Independence (AMI) which represents 750 municipalities.
"They can arrest us, they're crazy!," David Rovira, the pro-separatist mayor of L'Espluga de Francoli, a town of some 3,800 residents, told AFP by telephone, adding that Madrid had "proposed nothing" to appease Catalonia's demands for greater autonomy.
Joan Rabasseda, the separatist mayor of Arenys de Munt which in 2009 became the first town in Catalonia to hold a symbolic independence referendum, said he was simply "obeying" the law calling the referendum passed by Catalonia's regional parliament.
"Obviously I don't like these threats or that they say you will be arrested, I have a family but I also have a political commitment" with voters, he added.
The ruling comes a day after prosecutors ordered police in Catalonia to seize ballot boxes, election flyers and any other item that could be used in the referendum.
Prosecutors have already launched an official complaint against Catalan president Carles Puigdemont and members of his government over their referendum plans, accusing them of civil disobedience, misfeasance and misappropriation of public funds -- the latter carrying jail sentences of up to eight years.
Prime Minister Mariano Rajoy's conservative government has vowed to do everything in his power to stop the referendum. It argues Spain's 1978 constitution stipulates that regional governments cannot call an independence referendum.
Rajoy on Wednesday urged Catalans to boycott the referendum.
"If anyone urges you to go to a polling station, don't go because the referendum can't take place, it would be an absolutely illegal act," he said.
Spain's Constitutional Court has suspended a referendum law that was fast-tracked through Catalonia's regional parliament last week but the Catalan government has vowed to go ahead with the vote nonetheless.
Catalan authorities routinely ignore the court's decisions as they do not recognise its legitimacy.
In his first public comments since the Catalan government signed a decree calling the referendum, Spain's King Felipe VI said the rights of all Spaniards will be upheld against "whoever steps outside constitutional and statutory law".
But constitutional law professor Javier Perez Royo said it would be difficult for the state to stop the referendum if huge numbers of Catalans disobey the law.
"The repressive apparatus of the state works when disobedience is reduced, but when you have 600 municipalities, a regional government, other administrations, how do you do it?...the level of tension could be terrible," he told AFP.
Catalonia, which is roughly the size of Belgium and accounts for about one-fifth of Spain's economic output, already has significant powers over matters such as education and healthcare.
But Spain's economic worries, coupled with a perception that the region pays more in taxes than it receives in investments and transfers from Madrid, have helped push the cause of secession from the fringes of Catalan politics to centre stage.
Hundreds of thousands of Catalans rallied on Monday in Barcelona on their national day to demand their region's secession from Spain and support the right to vote.
One big banner at the rally read: "Goodbye Spain".
Opinion polls show that Catalans are evenly divided on independence. But over 70 percent want a referendum to take place to settle the matter, similar to the plebiscite held in Scotland in 2014.