Demonstrators gather as they protest at Sol square, in Madrid (Photo: AP)
Spaniard protesters, who blame politicians for bleak economic prospects and jobless rate of more than 21 percent, remained camped under plastic covers in central Madrid's Puerta del Sol square on election day.
The electoral impact of the swelling protests, which have brought tens of thousands into the streets since last weekend, including an estimated 60,000 on the eve of the election, was difficult to gauge.
More than 34 million people are eligible to vote Sunday, choosing 8,116 mayors, 68,400 town councillors and 824 members of regional parliaments for 13 of the 17 semi-autonomous regions.
Surveys forecast devastating losses for the Socialists as voters vent their anger over the loss of millions of jobs and painful spending cuts, including to state salaries.
They predict the ruling party of Prime Minister Jose Luis Rodriguez Zapatero will lose control of strongholds such as the cities of Barcelona and Seville, and in the central region of Castilla-La Mancha.
It could even fall short of an absolute majority in all the 13 regions at stake.
The predicted rout would be a bad omen for the party ahead of general elections scheduled for March 2012, when the conservative Popular Party is expected to romp back into office after eight years in opposition.
Campaigning and forecasts of the outcome have been overwhelmed and muddied by a nationwide protest against joblessness, the economic crisis, corruption and politicians in general.
Spain's press urged people to cast a vote, however, warning that disaffection with politicians should not lead to a rejection of democracy.
"The crisis favours a populist rejection of politics, but the solution is not to abstain," said an editorial in the leading daily El Pais.
The conservative daily El Mundo noted that many of the young protesters were not alive when Spain transitioned to democracy after the 1975 death of General Francisco Franco.
"Maybe they don't know what it cost to get here. Democracy was a collective achievement of millions of Spaniards and it delivered longed-for freedoms but also the greatest period of development and economic prosperity in history," El Mundo said.
The paper chastised the government for failing to attempt to disband the protesters after Spain's electoral commission ruled them illegal.
Thousands of people have massed in city centres across the country in the biggest spontaneous protests since the property bubble burst in 2008 and plunged Spain into a recession from which it emerged only this year.
The crisis pushed Spain's unemployment rate to 21.19 percent in the first quarter of this year, the highest in the industrialised world. For under-25s, the rate in February was 44.6 percent.
Zapatero has voiced sympathy with the protesters, saying they are merely reacting "in a peaceful manner" to the economic crisis, and his government has resisted ordering police to disperse them.
Much of focus on Sunday was also on the northern semi-autonomous Basque Country, where a new political force, Bildu, is taking on the traditional parties after a court battle to prove it is not a mouthpiece for armed separatist group ETA.
Polls predict Bildu will leap into third place in the Basque Country behind the Basque Nationalist Party (PNV) and the Socialists, beating the main conservative opposition, the Popular Party, in Sunday's vote.
Polls opened at 9:00 am (0700 GMT) and were to close at 8:00 pm, with first results expected between 10:00 and 11:00 pm.