"There are thousands of men and women coming from all corners of Spain, even from Peru, to tell the Catalans that they are not alone, that we are with them, that we are going to fight together for freedom", he added.
The most serious threat to Spain's national unity in decades is likely to come to a head in the coming days with attention focused on whether Catalonia will make good on its promise to declare independence - and what form that declaration may take if it comes to pass.
The vote was held without regular electoral lists or observers - and polls indicate Catalans are split on the prospect of independence.
Thousands of people are shutting down streets in one of Europe's biggest cities, marching with their hands painted white.More news: Who Is Lauren Sivan? Harvey Weinstein Allegedly Masturbated In Front Of Reporter
"A region newly-independent would, by the fact of its independence, a third country in relation to the Union and all the treaties no longer apply to its territory from the first day of its independence ", said Romano Prodi in 2004.
As NPR's Lauren Frayer reports from Barcelona, Catalonia's capital, the people in the region are almost evenly divided over the issue of independence even though many who are in favor of staying with Spain may not have voted in the referendum.
Sturgeon reiterated her call for dialogue between the Catalan and Spanish Governments in resolving the situation.
More than 500 people gathered on Sunday at midday, on the Esplanade du Parlement Européen in Brussels, in support of the anti-independence demonstrations in Barcelona.More news: Google Pixel 2 XL vs Pixel 2 vs Apple iPhone X
But uncertainty still haunts the country as Catalan leaders have said they could declare independence this week. Catalan president Carles Puigdemont has pledged to push ahead for independence. "We will do everything that legislation allows us to ensure this", Rajoy told the German newspaper Die Welt. However, some demonstrators carried signs urging Rajoy to trigger Article 155 of the constitution, the so-called nuclear option which could be used to revoke the region's autonomy.
The sides dug in as the clock ticked down to a Tuesday evening session in the regional parliament where separatists have called for an independence declaration, a plan that has raised concerns for stability in the European Union.
The region of Catalonia, home to 7.5 million people in the northeast, is crucial to Spain, which is the EU's fifth-biggest economy and a member of North Atlantic Treaty Organisation.
While Spanish premier Mariano Rajoy did not attend but expressed his support in a tweet, a number of heavyweights from the ruling Partido Popular, including the president of the Madrid region, Cristina Cifuentes, took part.More news: Joe Hart needs more nastiness as England goalkeeper - Stuart Pearce
Many in the crowd forming in a central square are carrying Spanish and Catalan flags.