The vote has not benefited from the guarantees usually required (electoral commission, assessors, electoral lists, public, secret ballot, etc.), making it unlikely the recognition by the 28 independence of Catalonia proclaimed on this basis.
Other demonstrations - including in the Catalan city Barcelona - have also been held urging political dialogue.
Leaders in Catalonia are facing increasing domestic and worldwide pressure to abandon plans to declare independence from Spain, ahead of a planned speech by Catalonia's regional president.
Hundreds of thousands of them took to the streets of Barcelona yesterday in protest at the Catalan Government's plan to announce an imminent secession from Spain.
Spanish Prime Minister Mariano Rajoy has vowed that his government will not allow Catalonia to break away from the rest of the country. "This crisis needs to be resolved through dialogue at all levels of Spanish politics", France's European affairs minister Nathalie Loiseau said.
March organisers Societat Civil Catalana claimed 930,000 people turned out, but Barcelona police put the total at around 350,000.
Rajoy has issued a stern warning to Catalan leaders who have said they could declare independence this week.
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Article 155 of Spain's constitution says that if a region's government breaches its constitutional obligations or "acts in a way that seriously threatens the general interest of Spain", Madrid can "take necessary measures to oblige it forcibly to comply or to protect said general interest".
"The turnout in Barcelona and across Spain to talk about Spain and unity and to express the voice of those who would not want this referendum to go ahead was also overwhelmingly powerful".
Artur Mas was the Catalan president between 2010 and 2015 and helped negotiate a new Statute of Autonomy with the Spanish Government before it was overruled by the Constitutional Court. "It is up to the government to make decisions, and to do so at the right moment", Rajoy said in an interview with German newspaper Die Welt on Monday.
Pro-independence leader Carles Puigdemont is coming under increasing pressure after a number of Catalan-based companies re-located outside of the region in the wake of an unofficial independence vote just over a week ago.
Catalonia, a northeastern region about the size of Belgium, is home to 7.5 million people and accounts for a fifth of Spain's economy.
Spain's President Rajoy has said he would use all lawful means to keep Spain intact.
Taken over by France in the 17th century, many in this region nestled between the Pyrenees mountains and the Mediterranean retain close cultural links with Catalonia to the south.