Many are carrying pro-independence flags and signs calling for the Oct. 1 vote that the Spanish government calls illegal and has pledged to stop, AP reported.
The crowds have been asked by secessionist politicians and grassroots groups to print and distribute posters supporting the vote.
Carme Forcadell, the speaker of Catalonia's regional parliament, told a Barcelona crowd, "I ask you to go out and vote! Vote for the future of Catalonia!"
Spain's Constitutional Court suspended the law calling for the referendum and police have cracked down on preparations for the vote.
Separatists have pressed ahead anyway, vowing to declare independence if the "Yes" wins.
