Prime Minister Mariano Rajoy appealed to Catalans to ignore calls from independence supporters to turn out to vote.
The Spanish attorney general's office said that out of 948 Catalan mayors, about 700 are under investigation for supporting the independence referendum scheduled for October 1, and those who don't appear at the prosecutor's office when called to do so could be arrested. "Defending democracy can never be a crime", tweeted the Association of Municipalities for Independence, the organization that published the list of pro-independence mayors the authorities are now investigating.
September 11 marks the 'Diada, ' Catalonia's national day, which commemorates the fall of Barcelona to Spain in 1714 and is traditionally used by pro-independence activists to call for secession for the northeastern region with a distinct language.
Catalonia's pro-separatist government has asked the northeastern region's 947 mayors to provide facilities for polling stations for the independence referendum.
On Wednesday, Rajoy implored people in Catalonia to refrain from taking part in the October 1 vote, saying it would be illegal.
Speaking at a ceremony to award national culture prizes, Felipe says the constitution "will prevail" against any attempt to break Spain apart.
The Constitutional Court has already suspended all legislation approved by the Catalan parliament aiming to lay the legal groundwork for the vote; judges are investigating regional President Carles Puigdemont and other officials on charges that could result in prison sentences; and prosecutors have ordered the police to hunt down ballot boxes and all materials related to the vote.
The court has since 2015 declared regional independence referendums to be unconstitutional.
The king added that the rights of all Spaniards will be upheld against "whoever steps outside constitutional and statutory law".
However, the pro-independence coalition ruling Catalonia has pledged to hold the vote despite the prohibition.
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.