Spanish voters on Sunday are heading out to polling stations for the second time in less than a month as the country holds regional, municipal and European Parliament elections all on the same day.

The full results of the voting are not expected until the early hours of Monday morning given the scale of the operation but political observers will be keeping a close track of the outcome to see whether the upset in the snap general election of Apr. 28 continues its trend.

Spain's acting Prime Minister Pedro Sánchez gambled when he called those early elections but it paid off as his Socialist Party (PSOE) emerged with 123 of the 350 seats in the lower chamber of Parliament, the Madrid-based Congress of Deputies, while the Popular Party, a long-time powerhouse of Spanish conservatism, tumbled in stature amid competition from center-right and far-right parties.

The 47-year-old PSOE leader was the first of the major political parties to come out and vote on Sunday morning.

"Today is a very important day. We participate to continue advancing in social justice, coexistence and democratic regeneration in municipalities, communities, and autonomous cities, across the continent," Sánchez said on Twitter. "Today we decide the future of Europe and Spain for the next few years."

Around 35.3 million people are eligible to vote in the country's municipal elections while 37.3 million can vote in the EU elections, given the register includes non-Spanish EU citizens living in Spain.

With voting open between 9am-8pm in 23,194 polling stations across the country, Spaniards will also elect aspiring mayors in 8,131 municipalities ranging from rural villages to major cities like Barcelona and Madrid.

As she cast her vote, Manuela Carmena, the incumbent Madrid mayor, said: "Today Madrid makes the best homage to democracy: to participate. No-one stay at home, Madrid is better when it can count on everyone. I've already voted."

Pollsters have tipped the PSOE to be the most voted party in big cities, like Valencia, Zaragoza, Seville and Malaga.

Some 12 of Spain's 17 autonomous communities are also up for grabs.

For the PP, the elections will come as a challenge to see whether they can hold onto power in the regional assemblies they currently control, such as the Community of Madrid, Castile and Leon, La Rioja and Murcia.

It remains to be seen, too, whether the conservative group, led by Pablo Casado, will crack deals with business-friendly Ciudadanos (Citizens, C's) and the far-right Vox, as it did in regional elections in Andalusia back in Dec. 2018 to force PSOE out of power.

PSOE will hope to ride its wave of popularity and maintain its dominance in regional assemblies in Aragon, Castilla-La Mancha, Extremadura and Asturias. The center-left group may also be capable of causing additional upset by pushing the PP farther to the sidelines.

Both the major parties in Spain will likely need to collaborate with smaller parties.

In Catalonia, the battlefield is a little different.

There are no regional elections this time around, but voters in Barcelona have been called to elect a mayor with the post being fought over by the incumbent, left-wing Ada Colau, and Ernest Maragall, a pro-Catalan separatist candidate.

The vote for the City Hall is a litmus test for political sentiment in Spain's second-largest city as the fallout of an illegal referendum in Catalonia back in 2017 continues to dog Spanish politics. Twelve leading Catalan politicians are currently on trial for their alleged involvement in the separatist bid.

Carles Puigdemont, the former regional president of Catalonia who lives in self-exile in Brussels to avoid charges of rebellion and sedition, is running in the EU elections.

Beyond Spain, Sánchez will look to cement his influence in the EU as Spain elects 54 members of the European Parliament.

Given the decline of center-left parties in countries like France or Germany, the Spanish PSOE could become the most important force in the EU's legislative chamber.EFE-EPA