Spain's central government and regional Catalan authorities argued on Saturday over who controls the regional police force that is considered key to the success of a planned independence vote for the north-eastern region.
The Catalan government has already vowed to push ahead with the referendum on October 1, which the Spanish government considers illegal, and on Saturday said it was refusing to hand over control of the Mossos d'Esquadra police force to Spain's Interior Ministry.
The ministry had announced earlier in the day it would begin co-ordinating all police efforts in Catalonia to crack down on preparations for the vote, including sending direct orders to the Mossos, as the north-eastern region's largest police force are often called.
The control of the Catalan police has become a sensitive topic as the political confrontation between the pro-independence regional government and central authorities has poured on to the streets of Barcelona and elsewhere in Catalonia.
There have been intermittent pro-referendum protests, at times by thousands of people, since a judge ordered raids on Catalan government offices and arrested a dozen officials on Wednesday.
All the arrested this week had been released by Friday, although six of them remain under investigation for allegedly helping in the logistics for the vote.
The Mossos have been criticised by unions and members of the national police bodies for not cracking down hard enough on the referendum.
"We denounce the attempt by the state to intervene in the police forces of Catalonia," Joaquim Forn, the head of Catalonia's interior department and the civilian head of the Catalan police, said Saturday, reading a statement on regional television.
"This is unacceptable."
Mr Forn said the top police officer in the Mossos d'Esquadra had expressed his opposition to the measure during a meeting Saturday with the top state prosecutor in Catalonia and chiefs of Spain's two national police forces, the National Police and the Civil Guard.
"We will continue working like we've done until now," said a statement posted on the Mossos official Twitter account.
"We will exercise our powers to guarantee security and public order and be at the service of citizens."
An Interior Ministry official said Saturday's measure did "not mean taking command" of the Catalan police, but that it was "simply to agree on a means of co-ordination".
Spain's post-dictatorship constitution only allows the central government to call a referendum on secession and establishes that all Spaniards must have a say in a vote on the country's sovereignty.
Prime minister Mariano Rajoy said his government is confident that there will be no referendum in Catalonia because "no democracy can accept the wiping out of the constitution and the national sovereignty".
Mr Rajoy is trying to crush the referendum without a heavy-handed approach that could exacerbate the separatist sentiment.
But this week's crackdown has already sent thousands to the streets in Catalonia, while Mr Rajoy has been targeted by his political opponents for not acting earlier with an offer of dialogue.
Polls show that support for secession in Catalonia rose to more than half of its 7.5 million residents at the height of the financial crisis that began in 2008.
That support has been waning in recent months as Spain's national and regional economies have begun thriving.