On the eve of the rally, Spain's Prime Minister Mariano Rajoy issued a stern warning to Catalan leaders who have said they could declare independence this week.
Some protesters called for the region's separatist president Carles Puigdemont to go to jail for holding a vote on independence in defiance of the Spanish government and courts.
In recent weeks, a stream of Catalonia-based firms and banks have moved their legal bases outside the regionas a crisis over a Catalonian push for independence from Spain deepened. "We will do everything that legislation allows us to ensure this", Rajoy told the German newspaper Die Welt.
Hundreds of thousands of people came out in Barcelona on Sunday to protest against Ctalonia's secession from Spain.
Separatist politicians say there will be a declaration of independence for the northeastern region of 7.5 million during that session, although some ruling coalition lawmakers say the move could be simply "symbolic".
"We have said yes to so many mediation options that have been proposed", he said.
"If there were a declaration of independence it would be unilateral and it wouldn't be recognised", Nathalie Loiseau said on CNews digital news channel.
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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.
Some 90% of Catalans who voted in a disputed referendum on Oct.1 backed independence.
Although he did not say which specific charges Mr Puigdemont might face, according to El Español, Mr Casado warned that in Spain the crimes of sedition carry a maximum prison sentence of 15 years and rebellion against the state 25 years.
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.
"Escalation will not benefit anyone", she said, urging both sides to begin dialogue with each other and for Rajoy to "be responsible and listen to the people", and to roll back the police force that was dispatched to the region ahead of the vote.
"Many people stayed home and didn't vote (Sunday) so the whole question of the mandate that he would have in speaking before that Parliament and declaring independence would be challenged". But it also found further backing on Monday when the opposition leader Pedro Sánchez, head of the Socialist party, finally confirmed that his party would "stand by the [Spanish] state" if a declaration of independence materialised.
But Rajoy assured Catalan leaders that there "is still time" to backtrack and avoid the imposition of direct rule from Madrid. Scenes of Spanish police beating unarmed voters in the October 1 referendum caused global shock.