The Spanish city's broad, tree-lined boulevards were a sea of yellow T-shirts that evoked the yellow-and-red striped Catalan flag. Many participants carried the pro-independence flag, known as the 'estelada,'' which also contains a blue triangle and a white star. The crowd passed a giant banner calling for a secession referendum overhead.
This year's annual celebration came amid growing excitement and tension over the independence vote planned for Oct. 1. Spain's constitutional court has suspended the referendum while it considers its legality, but Catalan leaders say they will go ahead with it anyway.
Spain's national government, based in Madrid, is doing all it can to stop the ballot, which it says is illegal. Catalan independence parties said Monday's huge turnout in the regional capital - estimated by Barcelona's municipal police at 1 million - was a show of strength that would add momentum to their cause.
'Today we have said loud and clear that no orders from any court will stop us,'' Jordi Sanchez, head of the grassroots movement Assemblea Nacional Catalana, said in a speech to the crowd.
While the standoff between Barcelona and Madrid is creating divisions, the good-humored celebration attended by families produced no signs of conflict
A woman takes photos from a balcony during march to celebrate Catalan National Day in Barcelona, Spain, Sept. 11, 2017.
Participants sang and clapped along to recordings of the Catalan anthem 'Els Segadors'' (The Reapers). At one point, the crowd shouted in unison: 'Independencia!'' - Independence! The symbolic moment came after organizers counted down over a public address system to 5.14 p.m., which on a 24-hour clock is 1714.
That's the year independence supporters regard as the point when Catalonia lost much of the self-governing power it enjoyed for centuries.
Among the comparatively wealthy region's grievances is that because it accounts for a fifth of Spain's economic output, it pays more into the central government's coffers than it receives.
Nuria Bou, who wore a pro-independence flag tied around her neck like a cape, said she hoped she would get a chance to vote.
'We don't have anything against Spaniards,'' Bou said. 'But for many years the Spanish government has been making cuts to the funds we receive, and what we want is to govern ourselves.''
Miquel Puig, 41, a pro-independence Barcelona resident who runs a language school, wore a T-shirt reading 'Ara es l'hora,'' which translates to 'Now is the moment.'' Puig said he was motivated by 'a mix of cultural, social and economic issues.''
He noted that Catalonia, with a population of 7.5 million, has its own language and culture, that Catalans feel ignored by authorities in Madrid, and that the region can stand alone financially.
In a proof of their commitment to holding the vote, Catalan officials on Monday said mail-in voting by Catalan expatriates had already started.
Most Catalans support a vote on whether the prosperous region's future lies within or outside of Spain, but polls show that a referendum approved by the central government is preferred over a vote Madrid opposes.
Citizens also are divided over the independence issue. According to a June survey by the Catalan government's own polling agency, 41 percent supported independence while 49 percent were for staying in Spain. Outside of Catalonia, most Spaniards reject the idea.
Castillo Cancho, 69 and retired, did not go to the city center to join in the traditional march. He complained that what was once a day to celebrate Catalan culture has been usurped by the separatist cause.
Cancho is not in favor of independence and embraces his dual identity of Spanish and Catalan, but even so, he hopes that the Oct. 1 vote is held.
'If they don't let them vote, I will be annoyed, and I would almost be pushed to go vote if I could,'' he said. 'Repression make you rebel.''
His wife Rosa Maria Descalzo, 60, was wary of the vote because of the lack of legal guarantees such as an official voter roll.
'I am not convinced by the reasons they are giving for independence,'' she said. 'When everyone is opening frontiers, why should we be closing them?''