About 30% of 700,000 eligible voters in 170 towns and villages in the region's Catalan-language-speaking heartland voted on the question of whether Catalonia should become an independent state within the European Union, organisers said.
This was below the 40% target initially mentioned by leaders of the campaign, which aims to put pressure on Catalonia's biggest political parties to call for a real referendum on secession in the future.
Early results showed almost 95% of those who voted wanted Catalonia, which has a population of seven million and already enjoys considerable autonomy, to leave Spain.
But, with separatists bound to be more likely to take part in a vote which the Spanish government had dismissed as illegitimate, organisers had been more interested in seeing the level of turnout.
The result of the vote, which was dismissed as futile by the Spanish government, will have no legal impact, but organisers hope to organise another, bigger referendum in the region's capital, Barcelona, next year.
"This has been a powerful event that is going to push us towards independence," referendum campaign spokesman Uriel Beltran told Reuters by telephone.
The campaign was organised by a coalition of Catalan nationalists, including left-wing parties and dissident members of Catalonia's biggest political group, the centre-right Convergencia i Unio. Their campaign has been well organised, but backers say they have had to mobilise voters with a fraction of the resources available for official elections.
Catalan regional elections are due by the end of 2010, and any surge in separatist sentiment would be a serious problem for Spain's Socialist prime minister, José Luís Rodriguez Zapatero, just when he needs to focus on dragging Spain from recession without unsettling debt markets.
There was a steady flow of voters throughout the day in the towns chosen for the vote, which included nationalist heartlands where the Catalan language is widely spoken instead of Spanish. Catalonia, which has a distinct cultural identity reaching back to the Middle Ages, already enjoys considerable control over its own affairs.
But separatist sentiment, which re-emerged after the death of dictator Francisco Franco in 1975, has been stirred by rumours that Spain's constitutional court will reject much of the region's new constitution, which includes a preamble defining Catalonia as a nation.
Sunday's vote moved the referendum campaign up a gear following its first foray in September, when more than 2,000 inhabitants of the small town of Arenys de Munt voted on the question, 96% of them favouring independence.
Opinion polls differ over the level of support for independence in Catalonia, where about half the population speaks Catalan at home instead of Spanish.
The Spanish government says the constitution would not allow a real referendum on regional independence.
(EurActiv with Reuters.)