Nobel Literature Prize laureate Mario Vargas Llosa said that about 930,000 people had gathered, adding that "for some time now, nationalism has been wreaking havoc in Catalonia and that's why we're here, to stop it".
The "Yes" side won the referendum by a landslide, winning 90 per cent of the vote, though less than half of the region's electorate cast their ballots.
Hundreds of people are gathering in downtown Barcelona ahead of a rally to protest the Catalan government's push for secession from the rest of Spain. "If tension persists, it could affect investments and tourism, slowing GDP growth more than we already forecast", he said in a note to clients.
Catalan leaders came under intense domestic and worldwide pressure on Monday to halt plans to break away from Spain after the region's president repeated his threat to declare independence and the government warned it would act to block it. According to France's European Affairs Minister Nathalie Loiseau, "This crisis needs to be resolved through dialogue at all levels of Spanish politics". In response to the vote, Spanish riot police shut down polling stations and beat protesters, leaving nearly 900 people injured.
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"I don't rule out anything", Rajoy said in an interview with the daily newspaper El Pais published October, 8 when asked about applying the constitutional provision that allows the suspension of autonomy and the imposition of direct rule from Madrid.
Until now, Rajoy has avoided saying whether or not he would use article 155 - the so-called nuclear option - of the constitution that allows him to sack the regional government and call a fresh local election.
In a manifesto read at the end of the demonstration, there were demands that non-nationalist Catalans "should not be marginalised" and Mr Vargas Llosa launched a stinging attack on the nationalists, accusing their leaders, amongst them regional premier Carles Puigdemont, of being golpistas - conspirators in a coup d'etat. Residents, some 7.5 million people, have also been able to maintain their national language: Catalan.
Meanwhile, the regional president of Catalonia, Carles Puigdemont, announced he would appear in the regional parliament on Tuesday to explain the political situation.
The Catalan government had previously said it would declare independence within 48 hours of a yes vote in the referendum. Spain's recent financial crisis and the harsh austerity measures that followed generated more support for secession.