The Spanish government would "take measures" if a unilateral declaration of independence takes place in Spain's northeastern region of Catalonia, Spain's Deputy Prime Minister Soraya Saenz de Santamaria said on Monday.
Catalan regional president Carles Puigdemont is thought to be ready to declare independence at a session of the region's parliament tonight.
Spain's Prime Minister Mariano Rajoy could suspend the existing autonomous status that Catalonia enjoys under the country's system of regional governments. "We will do everything that legislation allows us to ensure this", Rajoy told the German newspaper Die Welt.
However, the issue has divided the region and Spain as the unionists took to the streets this weekend to protest Catalonia's independence.
Benet Salellas of the separatist Catalan CUP party said: 'It's very clear to me that those who I represent won't accept any other scenario'. Its European affairs minister said, "This crisis needs to be resolved through dialogue".
"We have listened to many people".
"Credibility and dignity suggest making the declaration of independence tomorrow", said Jordi Sanchez, the head of the civil group National Catalonia Assembly said today.
Under Catalonia's referendum law, deemed unconstitutional by Madrid, a vote on Tuesday would start a six-month process that would envisage divorce talks with Spain before regional elections and a final act of separation.
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"The unity of Spain can not be voted on or negotiated - it must be defended", read one sign in the crowd.
The crowd waved both Spanish and Catalan flags and chanted "Viva Espana!"
Instead of immediately declaring independence after an outlawed referendum on October 1, Puigdemont has played for time by saying he is open to negotiations or mediation.
Catalonia accounts for almost a fifth of Spain's economy, and leads all regions in producing 25% of the country's exports, CNNMoney reports.
Catalonia's High Court asked for Spanish national police to provide extra security at the court building in the event of the Catalan Parliament declaring independence.
However, hundreds of thousands of people have turned out for anti-independence rallies, including in Catalonia's capital, Barcelona, since the vote. But uncertainty still stalks the country as Catalan leaders have not backed off from plans to declare the region independent. "It is up to the government to make decisions, and to do so at the right moment", Rajoy said.
Spain called the referendum illegal and police in riot gear moved in on the day of the vote to try to forcibly shut it down, firing rubber bullets at unarmed protesters.