Police say 540,000 people turned out in the capital of Catalonia, a region in north-east Spain, while four other Catalan towns also held demonstrations, BBC reported.
Regional president Carles Puigdemont had urged support for "converting Catalonia into an independent state". Spain has consistently blocked attempts to hold a referendum on secession.
Puigdemont, who took power in January, faces a confidence vote on 28 September. He advocates a negotiated withdrawal from Spain but has hinted that Catalonia might declare independence unilaterally if necessary.
Sunday is the "Diada", Catalonia's national day which commemorates the moment in 1714 when Barcelona fell during the War of the Spanish Succession, an event that marked its history.
Spain's leading parties, deadlocked in their efforts to form a new government after two inconclusive general elections since December, have shown little sympathy for Catalan grievances.
The region of 7.5 million people makes up 16 per cent of Spain's population and accounts for almost 19 per cent of national GDP.
