Voters formed long queues outside polling stations and some applauded as the ballots opened today morning, after weeks of tense legal wrangling with Spanish authorities.
One of Spain's richest but most indebted regions, Catalonia's long-standing yearning for greater autonomy has swelled in recent years of economic hardship, sharpened by resistance from Madrid.
"This is an opportunity we could not miss... We have been demanding it for a very long time," said Martin Arbaizar, 16, queueing under blue skies to vote at a polling station in a school in Barcelona.
"Even though it may not be official, I think the important thing is that they listen to us," said Arbaizar. "The more people vote and the more noise we make, the better."
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Spain's conservative Prime Minister Mariano Rajoy's conservative government has vowed to defend the unity of the country, as it recovers from a steep economic downturn.
His government has mounted a series of constitutional appeals to try to block the vote.
Rajoy says his country cannot hold an independence referendum like Scotland because, unlike Britain, it has a written constitution that forbids it.
He downplayed the significance of the poll at a party rally yesterday in the eastern city of Caceres.
"Call it whatever you like, but it is not a referendum, not a consultation, nor anything that resembles it," he said.
"What is certain is that it will not have any effect," he said.
Demands for greater autonomy there have been rumbling for years, but the latest bid by the region's president Artur Mas has pushed the issue further than ever before.
Catalonia took a step towards greater autonomy in 2006 when it formally adopted a charter that assigned it the status of a "nation".
But in 2010 Spain's Constitutional Court overruled that nationhood claim, fuelling pro-independence passions.