Voting is under way in Spain’s third general election in four years.
The election is marked by the rise of the far-right movement Vox, which opposes multiculturalism and has threatened to end self-rule for regions like Catalonia.
Socialist Prime Minister Pedro Sánchez has presented himself as a bulwark against the advance of the hard right.
Polls suggest his party will lead the vote but no single group will win an overall majority.
The final opinion poll in El País newspaper on Monday put the Socialists (PSOE) at about 30%, the conservative People’s Party (PP) at around 20%, the centre-right Ciudadanos and left-leaning Podemos both near 14% and the far right Vox party at about 11%.
However, the poll also showed that up to four in 10 voters had yet to make up their minds.
Voting began at 09:00 (07:00 GMT) and will end at 20:00 on the mainland.
An exit poll will be published when voting closes, but in the previous two elections this failed to give an accurate picture of the outcome.
Almost all votes are expected to be counted by midnight.
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Turnout stood at more than 41% as of 14:00, election authorities said – higher than the 37% recorded at the same point in 2016.
Catalonia in particular saw rise of nearly 11% in people coming out to vote.
What are the key issues?
The highly polarised campaign was dominated by issues including national identity, gender equality and the future of Catalonia.
The semi-autonomous region held an independence referendum in October 2017 and declared its independence from Spain weeks later.
A dozen of its leaders have since gone on trial in Madrid, facing charges including rebellion and sedition.
Analysts say support for Vox has been boosted by widespread anger at the independence drive. The party fervently opposes any concessions to the secessionists.
Women’s rights have also been a key campaigning topic. Gender-based violence has provoked debate and street protests across Spain for years and more politicians than ever are courting women’s votes.
Vox, however, has spoken out against what it calls “radical feminism” that it claims “criminalises” men.
What can happen next?
With polls suggesting there will be no overall majority, observers say the vote could plunge Spain into a prolonged period of political uncertainty as attempts are made to broker a coalition deal.
This also means that chances of a repeat election are high.
Particularly notable within the split vote is the rise in support for Vox, who are set to become the first far-right party to sit in parliament since 1982.
What are the candidates saying?
Speaking after casting his vote at a polling station near Madrid, Prime Minister Sánchez said he hoped for stability.
“After many years of instability and uncertainty, it’s important that today we send a clear, defined message about the Spain we want. And from there a broad parliamentary majority must be built that can support a stable government,” he told reporters.
Meanwhile, Albert Rivera, leader of the centre-right Ciudadanos, renewed calls to oust Mr Sánchez as he cast his vote in Barcelona.
“These are not any normal elections. At stake is whether we want to remain united, if we want to continue being free and equal citizens, if we want a Spain that looks to the past or the future, a country of extremes or of moderation,” he said.