Whether or not Catalan president Carles Puigdemont will follow through on his threat to announce a full breakaway – defying the central government and Spanish courts – is still a mystery.
But the Spanish government issued a sharp warning to Puigdemont on Tuesday as it grapples with the nation's worst political crisis in a generation.
"We call on Puigdemont not to do anything irreversible, not to pursue a path of no return and not to make any unilateral independence declaration," government spokesman Iñigo Mendez de Vigo told reporters.
Speaking soon after, Catalan government spokesman Jordi Turull retorted that the regional executive was "completely united", without giving any hint of what Puigdemont may tell Catalan lawmakers in an extraordinary parliamentary session beginning at 18.00 Spanish time.
'Rebellion against rule of law'
At stake is the future of a region of 7.5 million people deeply divided over independence, one of Spain's economic powerhouses whose drive to break away has raised concern for stability in the European Union.
Political leaders in Catalonia, Spain and Europe have urged Puigdemont to stand down and ease the country's biggest upheaval since its transition to democracy in the 1970s.
But the Catalan president says an independence referendum that took place on October 1st despite a court ban ruling it unconstitutional justifies splitting from Madrid.
Around 90 percent of those who cast ballots voted for independence but the poll was poorly monitored and many Catalans opposed to secession simply stayed at home. Turnout was just over 42 percent.
Spain's Economy Minister Luis de Guindos on Tuesday denounced the independence call as a "rebellion against the rule of law."
'The end of the road'
Catalan police were out in full force around the region's parliament in Barcelona ahead of Puidgemont's address.
"The end of the road," said Catalan daily El Periodico on its front page.
On Monday, Ada Colau, the popular mayor of Barcelona, warned that a unilateral declaration of independence would put "social cohesion" at risk.
The results of the referendum "cannot be an endorsement to proclaim independence but they constitute the possibility of opening a dialogue and international mediation", she said.
Pressure also came from the street itself, with hundreds of thousands of pro-unity demonstrators marching through Barcelona and Madrid at the weekend.
Their slogan, "Basta!", was simple: "Enough".
EU backs Spain
After the disputed referendum Puigdemont vowed he would declare independence in the coming days, but he has a variety of options to choose from.
Short of declaring an outright split, the Catalan leader could play for time and call for dialogue, or back down outright from his secessionist demands.
Madrid insists that any independence declaration would not change the legal reality that Catalonia is one of Spain's semi-autonomous regions with laws governed by the national constitution.
But EU nations are watching developments closely amid concern that Catalan independence could put further pressure on the bloc still dealing with the fallout from Britain's shock decision to leave.
After talks in Luxembourg with ministers from the European People's Party, the EU's right-of-centre political grouping, de Guindos said "everyone has supported the position of the Spanish government".
READ ALSO: Spain says it has EU support on Catalonia
'A disaster' for business
Prime Minister Mariano Rajoy has vowed to use everything in his legal power to prevent Catalan independence and has even refused to rule out imposing direct rule over the region from Madrid – a move many fear could lead to unrest.
The crisis has caused deep uncertainty for businesses in one of Spain's wealthiest regions.
A string of companies have already moved their legal headquarters – but not their employees – from Catalonia to other parts of the country.
The head of Spain's chamber of commerce Jose Luis Bonet told Cadena SER radio that a unilateral independence declaration "would be a disaster".
"For Spain it would be extraordinarily negative and even for Europe it would mean enormous instability," Bonet said.
Demands for independence in Catalonia, which has its own language and cultural traditions, date back centuries.
But a 2010 move by Spain's Constitutional Court to water down a statute that gave Catalonia additional powers, combined with an economic crisis in Spain, sparked a surge in support for independence.