Polling stations opened under cloudy skies in Barcelona, where red- and yellow-striped Catalan flags hung from buildings, AFP reporters saw.
More than 5.5 million of Catalonia's 7.5 million inhabitants were eligible to vote at nearly 2,700 polling stations across the region.
A pro-independence alliance led by regional president Artur Mas has vowed to proceed towards a declaration of independence by 2017 if it secures a majority in the regional parliament, even if it manages to do so without a majority of votes.
Spain's central government brands secession illegal and has called for the country to stay united as the eurozone's fourth-biggest economy recovers from recession.
Madrid says Catalonia would drop out of the European Union and eurozone if it broke away from Spain.
"Catalonia decides its future in Europe," ran Sunday's front-page headline in the centre-right national daily El Mundo.
"The future of Catalonia is at stake," said Catalan daily La Vanguardia.
Centre-left national El Pais declared the ballots "historic" on its front page.
Nationalists in Catalonia, which has its own language and cultural traditions, complain that they get less back from Madrid than they pay in taxes.
Separatist demands have surged in the recent years of economic crisis.
Mas wants Catalonia to follow the example of Scotland and Quebec in Canada by holding a vote on independence -- though in both those cases most voters rejected a breakaway.
Since Madrid has blocked Mas's efforts to hold a straight referendum, he has framed Sunday's election for the regional parliament as an indirect vote on secession.
Madrid has garnered support in the dispute from world leaders such as US President Barack Obama and British Prime Minister David Cameron, who have defended the unity of Spain.