Spain's government on Thursday set in motion plans to take away Catalonia's local powers after its defiant regional president refused to give up his demands for Catalan independence.

Carles Puigdemont sent a letter to Spanish Prime Minister Mariano Rajoy just minutes before a deadline set by the central government for him to backtrack on his calls for secession. Puigdemont didn't give in, but instead threatened to go ahead.

"If the state government persists in blocking dialogue and the repression continues, the parliament of Catalonia will proceed, if deemed appropriate, to vote on the formal declaration of independence," Puigdemont's letter said in an English translation provided by the Catalan regional government.

Spain's government responded by calling a special cabinet session for Saturday, during which it would trigger the process to activate Article 155 of Spain's 1978 Constitution. The article allows for central authorities to take over the semi-autonomous powers of any of the country's 17 regions, including Catalonia.

The 'nuclear option'

Regarded as the "nuclear option," such a punitive measure takes the standoff to another level. It probably will trigger outrage in Catalonia and could backfire by fostering sympathy for the independence movement, which polls suggest is supported by about half of Catalans.

The constitutional law has never been used in the four decades since democracy was restored at the end of Gen. Francisco Franco's dictatorship.

Spain's government needs to outline the exact measures it wants to apply in Catalonia and submit them for a vote in the Senate.

The ruling Popular Party's majority in the top chamber would be enough to approve the measure, but Rajoy has held discussions with opposition leaders to rally further support.

The main opposition Socialist party backed Rajoy's moves, but wants the Article 155 measures to be limited in scope and time.

Puigdemont

Catalan leader Carles Puigdemont, left, says he is prepared to proceed toward full independence if the Spanish government takes away the region's autonomy. (Manu Fernandez/Associated Press)

The article leaves it up to the national government to decide what specific measures to take. Officials say Madrid will almost certainly seize control of Catalonia's regional police to ensure law and order is maintained, along with tightening its grip on the region's finances.

Other measures being mulled are removing Puigdemont's presidential powers, rescinding regional control over education and schools, calling fresh elections that would dissolve the regional parliament, and taking control of public media that are seen as mouthpieces for Catalonia's pro-independence ruling coalition.

The dispute is increasingly encroaching on the European Union's political agenda. Catalonia wasn't officially to be discussed at an EU summit starting Thursday in Brussels, but leaders offered their views. French President Emmanuel Macron reiterated his recent support for Rajoy, saying that the summit would be "marked by a message of unity around member states amid the crises they could face, unity around Spain."

European Council President Donald Tusk ruled out any EU role in the dispute, telling reporters on the sidelines of the summit that "there is no room, no space for any kind of mediation, or international initiatives or action."

Belgium EU Brexit

Spanish Prime Minister Mariano Rajoy arrives for an EU summit in Brussels on Thursday. Rajoy has held discussions with opposition leaders to rally further support to activate Article 155 of Spain's 1978 Constitution. (Olivier Matthys/Associated Press)

Puigdemont addressed the regional parliament on Oct. 10, saying he had the mandate under a banned Oct. 1 referendum to declare independence from Spain. But he immediately suspended the implementation of the secession proclamation and called for talks with Spain and international mediators.

But Spain's government responded by setting two deadlines for Puigdemont — a Monday one for him to say a simple yes or no to whether he indeed had declared independence or, and a second one for Thursday morning for him to fall in line with Spain's laws.

Spain's government says Puigdemont hasn't offered any clarity in his replies.

Catalans would consider the application of the measure an "invasion" of the region's self-government, while Spain's central authorities have portrayed it as an undesired move, yet a necessary one, to restore legality after Puigdemont's government pushed ahead with a banned referendum that violated the country's Constitution.

SPAIN-POLITICS/CATALONIA-ARRESTS

Catalan leaders say they will declare independence if the Spanish government goes ahead with threats to take control of the region's semi-autonomous powers. (Albert Gea/Reuters)

Andrew Dowling, an expert in Catalan history at Cardiff University in Wales, said any declaration of independence in the Catalan parliament would be symbolic without border and institutional control and no international support.

Such a declaration "will see the fracture between hardliners and the pragmatic people in Catalonia who are already seeing an economic fallout," Dowling said.

Spain's Association of Commercial Registers said Thursday that 971 companies, including Catalan banks, multinationals and midsized businesses, have moved their registered addresses out of the troubled region because of concerns about its future.

Civil society groups that have drawn hundreds of thousands to the streets in peaceful pro-independence demonstrations over the past few years are calling for new protests Friday.

With files from Reuters