As Madrid prepared to pass measures by the weekend to remove Catalonia's leadership, far-left separatists were expected to detail their planned response if the central government moves to take over powers from the regional government.
The worst political crisis in Spain in decades was sparked by a banned October 1st independence referendum deemed illegal by the country's government and courts.
Based on the outcome, Catalonia's conservative regional president Carles Puigdemont initially threatened a unilateral declaration of independence. Madrid insisted it would take over the region's governance, and cash, to prevent that.
Puigdemont could call elections for a new regional parliament to stave off Madrid's seizure of power.
But a central government minister warned on Tuesday that elections might not be enough to prevent Madrid taking over the region.
As the nation waited to see who will blink first in the constitutional standoff, Spain's justice minister urged Puigdemont to clarify his stance.
"Mr Puigdemont's violation of his obligations cannot be resolved merely by calling elections," Rafael Catalá told RNE radio.
This would require "elections and something extra" -- which would include ruling out any possibility of unilaterally declaring independence.
The Senate was set to approve a formal mandate Tuesday for a 27-member committee that will examine how best to take over the running of Catalonia.
Meanwhile, the far-left, anti-capitalist Popular Unity Candidacy (CUP), scheduled a press conference in Barcelona to announce its planned reaction if the government goes ahead and executes an article of the constitution designed to rein in the regions.
The CUP will also hold meetings in a dozen cities and towns to plot a way forward.
On Monday, the party accused Madrid of the "biggest assault" on the Catalan people since Francisco Franco's dictatorship. It said a takeover by Madrid would be met with "massive civil disobedience".
Franco -- who ruled from 1939 until 1975 -- suppressed Catalonia's autonomy, language and culture.
Catalan firefighters, teachers and students have also warned of strikes and protests.
Under the never-before-used Article 155, Madrid could depose the regional government, wrest control from the Catalan police force, replace the heads of its public news outlets, and take over Catalonia's finances.
Until now, the region controlled its own policing, education and healthcare, but discontent has grown since the 2008 economic crisis, with Catalans demanding more control over their own tax income.
About 90 percent of participants voted for independence in the October 1st referendum, according to Catalonia's government, the Generalitat.
Only about 43 percent of eligible voters turned out however, with many anti-secessionists staying away.
The region of 7.5 million people is fiercely protective of its culture, language and autonomy, though polls indicate its inhabitants are deeply divided on whether to break away from Spain.
The Senate committee to be formalised on Tuesday includes representatives of governing, opposition and separatist parties.
It will send a formal notice to Catalan leader Carles Puigdemont to inform him of the measures, and give him an opportunity to respond -- possibly at a full session of the Senate to be held at the end of the week.
He can reply in writing, send a representative, or go to Madrid himself. A Catalan government spokesman said Puigdemont is mulling his options.
"If he responds by reiterating his positions on Catalan independence... unfortunately we cannot but apply the measures foreseen by the government," said Catalá.
The Article 155 measures would come into effect once published in the government gazette on Saturday, effectively ousting Puigdemont and his team.
Catalonia's separatist parties intend to hold a special session of the regional parliament for Thursday to decide how to respond.