People cast their votes during Spain's general election in a polling station in Barcelona, Spain, Sunday, April 28, 2019. Photo: AP Photo/Emilio Morenatti.

Madrid - Voting was underway across Spain on Sunday in a snap election called by Prime Minister Pedro Sanchez after Catalan parties withdrew their support for his budget proposal.

Nearly 37 million Spaniards are eligible to vote in the general election - Spain's third in four years.

Sanchez's Socialist Workers' Party (PSOE) - which has positioned itself as a bulwark against the rise of the far-right Vox party - is leading in the polls but likely to fall short of a majority.

It would need the backing of the centre-left Podemos and possibly other parties in order form a new government.

If conservative forces spearheaded by the People's Party (PP) manage to garner a majority, newcomer party Vox could become a member of a coalition. It would be the first time a far-right party has been in a central government for decades.

Vox is already part of the southern region of Andalucia's coalition. PP and the liberal Ciudadanos struck a deal with the party in February.

According to the last opinion poll published by El Pais newspaper, the PSOE is set to win 30% of the vote, while PP will win only 20%.

The poll puts the centre-right Ciudadanos and Podemos at 14%, respectively, while Vox comes in with 11% support.

The next regular parliamentary election would have been due in June 2020.