The ballot will result in a hung parliament, with parties favouring unity with Spain tipped to gain a maximum of 62 seats and pro-secession factions 63.
Spain’s Rajoy would probably call a snap regional election after activating Article 155 of the constitution that would allow him to sack the Catalan regional government.
In Madrid, some socialists have suggested Basque could serve as a model for a compromise that would defuse Spain’s biggest political crisis since a failed coup in 1981.
A majority voted for the referendum and the legal framework to set up a new state, under which the assembly would declare independence within 48 hours of a “yes” vote.
Previous secessionist challenges in Catalonia were blocked by Spain’s conservative government and the Constitutional Court.