A divided Spain is voting in its third general election in four years, with all eyes on whether a far-right party will enter Parliament for the first time in decades and potentially help unseat the Socialist government.
The incumbent prime minister, Pedro Sánchez, is set to win the most votes, but his Socialists seem far from scoring a majority in parliament to form a government on their own.
The fragmentation of the political landscape is the result of austerity that followed a recession, disenchantment with bipartisan politics and the recent rise of far-right populism.
Sánchez called Sunday's ballot after a national budget proposal was rejected in the Lower Chamber by the center-right-conservative opposition and Catalan separatists pressing for self-determination in their northeastern region.
Polls a week ago found that about one third of the nearly 37 million eligible voters hadn't decided how they would vote. Their decision, and the expected high turnout, could swing the result between the left and right wing blocs that have taken shape during the electoral race.
The anti-austerity Unidas Podemos (United We Can) party, has offered to enter a coalition with the Socialists, but they might need to rely on smaller parties, including the Catalan separatists.
On the splintered right, three parties are competing for leadership: the once-dominant conservative Popular Party, the center-right Citizens, and the nationalist and anti-migrant Vox party, which looks set to enter the lower house of Parliament for the first time. Its arrival would mark a big shift in Spain, where the far right has not played a significant role since the country's transition to democracy following the death of dictator Gen. Francisco Franco in 1975.
Voting stations opened at 9 a.m. (0700GMT) Sunday and will close at 8 p.m. (1800GMT), with results expected a few hours later.
Speaking Sunday shortly after casting his ballot, Sánchez said he wanted the ballot to yield a parliamentary majority that can undertake social and political reforms in the country.
The prime minister said he wanted the lower house to support "a stable government that with calmness, serenity and resolution looks to the future and achieves the progress that our country needs in social justice, national harmony and political cleansing."
Citizens leader Albert Rivera, who has focused his campaign on unseating the incumbent Socialist, told reporters in a town near Barcelona where he cast his ballot that a high turnout is needed for a government change and to "usher in a new era."
United We Can party leader Pablo Iglesias also stressed the importance of voting on Sunday.
"My feeling is that in Spain there is an ample progressive majority, and when there is high participation that becomes very clear," Iglesias told reporters at a public school in the residential suburb near Madrid where he lives.
Up for grabs are the 350 members of the Congress of Deputies, who then choose a government, and also 208 senators for the Upper House.
For the first time since Spain transitioned to democracy in the 1970s, more than 100,000 people with mental disabilities are allowed to vote in the general election.