Some European officials are also anxious that any softening in Spain's stance towards Catalan independence could fuel secessionist feelings among other groups in Europe such as Belgium's Flemings and Italy's Lombards.
A man holds up a sign during a march in downtown Barcelona, Spain, to protest the moves for Catalan independence. The crowd bristled with Spanish flags.
"Spain will continue being Spain".
"I don't have much confidence in the government". "And that would be very risky for the government of Catalonia, because it does not have majority support among the population", he adds.
The Spanish government sent thousands of national police into the region to prevent the vote.
But uncertainty still haunts the country as Catalan leaders have not backed off from plans to declare the region independent.
In Madrid thousands gathered beneath the enormous Spanish flag in Colon Plaza waving their own flags, singing and chanting "Viva España" and "Viva Catalonia".
We feel both Catalan and Spanish.
"There is a lot more tension and violence".
Mr Vargas Llosa is also among 60 Spain-based intellectuals and academics who have signed an open letter asking the worldwide community not to support the idea of external mediation as a solution to the crisis in Catalonia.
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"There has to be a commitment to dialogue", Jordi Cuixart, president of one of the grassroots groups driving the separatist movement, said on Catalan radio. They don't want to talk, particularly Madrid does not want to talk.
Opponents of secession called for demonstrations around Spain on Saturday and in Barcelona on Sunday.
But Sunday marks the second consecutive day of protests in Spain, with thousands marching on Saturday, calling for dialogue to defuse the tense situation.
"This is producing a social rupture in Catalonia and this has to be resolved through dialogue, never via unilateralism", Jose Manuel Garcia, 61, an economist who attended the protest dressed in white said.
"Everyone has the right to say their opinion. I think that whatever they do, they have made us angry, and I think we have seen that a. part of Spain doesn't like us, the King doesn't like us either and so I think that, also because of how they've treated us just now, there is no turning back".
The stark warning came days before Catalan leader Carles Puigdemont is expected to address the region's parliament, on Tuesday, when he could unilaterally declare independence.
He added: "I want to say something with absolute clarity - while the threat of independence is in the political landscape, it will be very hard for the government to not take these decisions".
On Friday, Catalonia's police chief and two prominent separatist leaders including Cuixart avoided being remanded in custody at a court hearing over sedition accusations.
Spanish Prime Minister Mariano Rajoy has offered all-party political talks to find a solution, opening the door to a deal giving Catalonia more autonomy, but only if the Catalan government gives up any independence ambitions.