Polls take place after parties failed to agree on a coalition government in December 2015.
Spain returned to the polls on Sunday for the second time since December 2015.
The elections are aimed at unblocking six months of political paralysis. More than 36 million voters are called on to cast their ballot until 1800 GMT to say if they want radical change as promised by a far-left coalition led by political party Podemos, or stability and safety as pledged by the outgoing conservative Popular Party (PP).
Why is another vote needed?
After four years in power, PP collected most votes in December's ballot but didn't elect enough lawmakers to give it a majority in Parliament, like it had before.
Mariano Rajoy, the party's leader and incumbent caretaker prime minister, couldn't get enough support from rival parties to form either a minority government or a coalition. The negotiations between parties dragged on for months as Pedro Sanchez, leader of the second-placed Spanish Socialist Workers Party, also failed to clinch a deal that would let him govern.
Unlike other European nations, Spain has never had a coalition government.
What's driving the Spanish campaign?
Europe still bears the scars of the Great Recession of 2007. In Spain, that means 20 per cent unemployment and reduced government spending on national health care and public education. On top of that, political corruption has angered many Spaniards.
Spain emerged from recession in late 2013 and is now one of the European Union's fastest growing economies. But there's not a significant trickle-down effect yet, and the unemployment rate remains the second-highest in the EU after Greece.
The PP has also been engulfed by corruption scandals in recent years, with jailed party treasurer Luis Barcenas telling investigators about a scheme of illegal contributions and donations to the party.
The Socialists are fighting smaller scandals of their own, involving allegations that former party members ran a fraud scheme by syphoning off public funds.
The elections come four days after the United Kingdom voted in a referendum to leave the European Union. Antonio Barroso, a London-based analyst with Teneo Intelligence political risk consultancy, said it is "unlikely" that decision would have much of an influence on the Spanish election.
"Nevertheless, the ongoing market turmoil fits well with the campaign message of ... Rajoy, who has framed the election as a choice between economic stability and a radical left-wing government potentially led by Podemos and its allies," Barroso wrote in an analysis on Friday.
What could happen with the vote?
Opinion polls suggest the new ballot won't break Spain's political stalemate. The polls indicate PP will again capture most votes but will still fall short of a parliamentary majority, returning Spain to the limbo as the parties renew negotiations.
According to the Spanish Constitution, a government must win a vote of confidence in Parliament with more than 50 percent of the possible 350 votes before taking office. If it misses that target, in a second vote 48 hours later it must get 50 percent of only the votes that are cast - a lower bar which allows parties to abstain from the vote and let a party into power in return for concessions.
One new factor could make a difference: the far-left Unidos Podemos (United We Can) alliance, which partners Podemos and the much smaller United Left party.
Polls suggest Unidos Podemos could secure second place, overtaking the centre-left Socialists, with the centrist, business-friendly Ciudadanos in fourth place.
The Socialists and Unidos Podemos could potentially create a broad left-of-centre coalition.
Analysts are not expecting a high turnout, due Spaniards' disillusionment with squabbling politicians.
After the election, King Felipe VI will begin talks with Spain's political leaders, searching for a candidate to form a government.