Friday's appeal came as Madrid moved to prevent state funds from being used to carry out a vote that the federal government maintains is illegal.

The first public crackdown on the process came as police broke up a meeting of a pro-independence Catalan party.

It was against that backdrop that Catalonia's regional president Carles Puigdemont and Barcelona Mayor Ada Colau — who are pushing for the separatist vote slated for October 1 — wrote to both Spanish Prime Minister Mariano Rajoy and to Spain's King Felipe VI, saying the state had launched "an unprecedented repression offensive."

Watch video 00:34 Catalan parliament separatists sing

The letter called for "political dialogue, based on the legitimacy we all have, to make possible something that in a democracy is never a problem and even less a crime: listening to the voice of the people."

The government said it had not received the letter.

"To talk about dialogue when the only thing the Catalan government has wanted for months, years, is a referendum come what may, seems like a threat," government spokesman Inigo Mendez de Vigo told a news conference.

Madrid asserts itself

The government gave itself new authority on Friday to increase its control over how the regional government spends its money. The move comes as Madrid tries to prevent state funds from being used to pay for the referendum, which will likely cost millions of euros.

The Catalan government envisions the referendum to be held similar to a regional election. During the last regional vote in September 2015, some 8,177 polling stations were set up across the region.

Today, among Catalonia's 948 municipalities, more than 700 have pledged to provide locations for polling stations. Many of them are small towns. But several large cities have refused to participate.

Spain's central government has warned Catalan mayors that it is their duty to "prevent or block any initiative related to the organization of the referendum."

Pro-separatist members of the Catalan parliament applauded after approving the independence referendum last week

Catalan officials have had a difficult time producing ballots for the referendum. They claim police have searched companies suspected of producing them.

Despite the problems, Puigdemont declared at the end of August that his government had over 6,000 ballot boxes ready for the referendum. He would not say where they were being stored for fear Spanish authorities would seize them.

bik/cmk (Reuters, AP, AFP)