Catalonia is holding an informal poll on independence.
The Spanish judiciary has ruled the vote unconstitutional but Catalan leader Artur Mas warned against any attempt to disrupt it.
Spain’s constitutional court suspended earlier plans for a referendum on secession.
PM Mariano Rajoy said the vote would have no effect and urged the region to return to “sanity”.
Voters will be asked whether they want a Catalan state and whether that state should be independent.
Catalonia is a wealthy a region of 7.5 million people and contributes more to the Spanish economy than it gets back through central government funds. Economic and cultural grievances have fuelled Catalan nationalism.
He says there is a long history of support for winning independence from Spain, or at least much greater autonomy within it.
This week, the Constitutional Court demanded the vote be suspended.
Catalonia’s government insisted it went ahead, organized by volunteers and with no official electoral roll.
Artur Mas warned the Spanish government against any attempt to halt the vote.
He said: “I don’t know what they will do, it does not depend on us, but if they have a minimum of common sense I think any action out of the ordinary would be a direct attack on democracy and a direct attack on fundamental rights.”
Mariano Rajoy urged a return to sanity and for talks “within the legal framework of the constitution”.
He said the vote would be “neither a referendum nor a consultation nor anything of the sort”.
He added: “What is certain is that it will not have any effect.”
The Libres e Iguales (Free and Equal) group, which opposes the vote, held protests in dozens of cities.
One protest in Barcelona witnessed minor scuffles but no arrests.
Rallies in favor of the vote have also been held.
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