One of Spain's biggest and richest regions, Catalonia, is holding a much-disputed symbolic vote on whether it should break away as an independent state.
Spanish Prime Minister Mariano Rajoy's conservative government has vowed to defend the unity of the country, and has mounted a series of constitutional appeals to try to block the vote.
However, Catalans have pushed ahead defiantly, fired up by the independence referendum held in Scotland in September, even though Scots voted not to break away from Britain.
Mr Rajoy said 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.
"What will take place tomorrow, we can call it whatever one wants, but it is not a referendum, not a consultation, nor anything that resembles it. I can't even qualify it.
"What is certain is that it will not have any effect," he said.
Proud of its distinct language and culture, Catalonia, a region of 7.5 million people, accounts for nearly a fifth of Spain's economy.
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.
Spain's recent economic crisis has increased unemployment and hardship in the region and swelled its debts, but in 2012 Mr Rajoy rejected Mr Mas's request for greater powers for Catalonia to tax and spend.
In response, Mr Mas vowed to hold an official yet non-binding vote on independence, but the Spanish government's legal challenges forced him to water that down.
The ballot papers will put two questions to voters: "Do you want Catalonia to be a state?" and if so, "Do you want that state to be independent?"
Ballot boxes were set up yesterday at schools and town halls across Catalonia, even though the central government has warned the regional government that it cannot use public resources to stage the vote.
The Spanish government has not specified what legal consequences, if any, Catalan leaders, volunteer poll workers or voters might face when they go to vote.
The regional government says results are expected tomorrow.