Pro-independence campaigners, including prominent citizens' group the Catalan National Assembly (ANC), said they had collected some 30,000 flags to hand out ahead of the Champions' League game pitting Barca against BATE Borisov of Belarus.
UEFA has already levied two fines this year after fans waved the starred pro-independence "Estelada" flag at various games, which the European football body says flout its rules on political messages at sport events.
"People should have the right to express themselves," said Barca fan Javier Guetard, as supporters began to stream into the into the Camp Nou stadium. People later whistled in protest as the UEFA anthem rang out.
"UEFA, we love our symbols," read a slogan Tweeted by pro-secession campaign groups on Wednesday, along with a photo of a smiling child holding up the flag.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Barca, with supporters around the world, has long been an outlet for those who favour secession in the wealthy northeastern region to express their beliefs, and especially since a recent Spanish recession has fed a belief that Catalans would be better off running their own economy.
A political platform advocating a split from Spain won a majority of seats in a September regional election, though its position has been fiercely opposed by the central government and it fell short of half the votes.
A former ANC leader, Carme Forcadell, was chosen to head Catalonia's parliament last month, and pro-secession groups have put forward a motion which they say will start an 18-month roadmap towards independence if it gains approval from lawmakers.
Spain's constitution does not allow a region to break away, however, and the governing centre-right People's Party (PP), which faces a tough general election in December, has dismissed any idea that independence would be feasible.
Earlier on Wednesday, the PP, opposition Socialists and newcomer party Ciudadanos clubbed together to ask Spain's Constitutional Court to block the motion on secession.
Barca has been broadly sympathetic to the independence cause, though it has steered clear of any major involvement in the secession campaign.
The club has vowed to fight the latest UEFA fine of 40,000 euros imposed after a September Champions' League game, adding it would not limit people's freedom of expression.
(Reporting by Tim Hanlon, Writing by Sarah White; Editing by Julien Toyer/Ruth Pitchford)
