Spain's state prosecutor is investigating more than 700 Catalan mayors for cooperating with a referendum on independence that has been suspended by a court.
The prosecutor has ordered police to arrest the mayors if they do not comply.
The pro-independence coalition ruling Catalonia has vowed to hold the vote despite the prohibition and has asked the 947 mayors in the northeastern region to provide facilities for the plebiscite.
Spanish prime minister Mariano Rajoy's conservative government has pledged to stop the referendum and was granted a suspension by the Constitutional Court while judges decide on its legality.
The country's top prosecutor, Jose Manuel Maza, ordered provincial prosecutors to investigate 712 mayors who have already offered municipal facilities for the October 1 vote and the regional Catalan police to arrest them if they do not show up to give evidence.
Spain's prime minister has urged the people of Catalonia not to take part in a planned referendum on the region's independence that he says is unconstitutional.
Mariano Rajoy is fighting to stop the ballot and he appealed to Catalans to ignore calls from independence supporters to turn out.
Mr Rajoy said: "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."
Spanish King Felipe VI commented for the first time on the political crisis triggered by Catalonia's plan to hold a vote, saying people must respect the country's constitution, which forbids secession.
Speaking at a ceremony to award national culture prizes, Felipe said the constitution "will prevail" against any attempt to break Spain apart.
He said the rights of all Spaniards will be upheld against "whoever steps outside constitutional and statutory law".