Spanish prime minister Mariano Rajoy warned that Spain will not be divided by a declaration of independence from Catalonia and said the government is ready to respond to any such attempt.
Catalan regional president Carles Puigdemont plans to address the region's parliament on Tuesday evening to debate the current political situation.
Separatist politicians said there will be a declaration of independence for the northeastern region of 7.5 million during that session, although some ruling coalition politicians say the move could be simply "symbolic."
Still, Mr Rajoy was being as explicit as possible in warning on Monday that the national government in Madrid would not stand for such a declaration.
"Spain will not be divided and the national unity will be preserved. We will do everything that legislation allows us to ensure this," Mr Rajoy told the German newspaper Die Welt.
"We will prevent this independence from taking place."
Secession-minded authorities in Catalonia have vowed to break away from Spain after claiming victory in a disputed independence referendum earlier this month.
The October 1 vote has been followed by mass protests of Catalans angered by police violence as authorities tried to stop the vote and, more recently, by others in Catalonia and Madrid urging the unity of Spain.
Yet politicians supporting Mr Puigdemont's minority government and civil society groups backing independence say they will not accept anything less than a full declaration of independence.
"Credibility and dignity suggest making the declaration of independence tomorrow," Jordi Sanchez, the head of the civil group National Catalonia Assembly, said on Monday.
A politician with the Catalan CUP party said the far-left separatists will not accept anything short of a declaration of secession.
"It's very clear to me that those who I represent won't accept any other scenario," Benet Salellas said during an interview at the regional parliament.
Mr Puigdemont has not clarified what his intentions are.
Mr Rajoy's deputy, Soraya Saenz de Santamaria, also warned that Spain would act decisively if there was any independence declaration.
"If they declare independence, there will be decisions to restore the law and democracy," she said on Monday during a radio interview.
She called for members of the Catalan government "who still respect democracy and freedom to refrain from jumping into the void."
Catalonia's top judicial official, meanwhile, ordered additional Spanish police protection for the headquarters of the regional judiciary.
The regional Mossos d'Esquadra police force, whose hierarchy reports to the Catalan government, had been in charge until now of guarding the palace in central Barcelona that hosts the judiciary.
But the High Judiciary in Catalonia says its president, Jesus Barrientos, has asked the chief of the National Police force in the region to join in the protection of the building.
The statement says a declaration of independence, even if illegal under Spanish laws, could trigger the suspension of the judiciary and the ousting of its president.