Right-wing parties have the most votes combined, though no majority.
This time Vox could double that number, polls suggest.
Spain's acting Prime Minister and Socialist Party (PSOE) leader and candidate Pedro Sanchez talks to media as he attends voting during Spain's general election in Pozuelo de Alarcon, outside Madrid, Spain, November 10, 2019.
Instead they show Socialists in the lead again, but with fewer votes than in April's election, and the conservative Popular Party (PP) and Vox making gains.
Many right-wingers were also not pleased by the Socialist government's exhumation of late dictator Gen. Francisco Franco's remains last month from his gargantuan mausoleum so he could no longer be exalted in a public place. But Vox leader Santiago Abascal said he would now work to build what he called "a patriotic alternative" for Spain.
It suggested far-right Vox becoming a third-biggest party in the parliament.
The Catalan crisis has dominated the election campaign, with parties on the right - Vox, the PP and the centre-right Ciudadanos - taking a hardline anti-separatist stance.
The far-left United We Can, which had rejected an offer to help the Socialists form a left-wing government over the summer, lost some ground and ended with 35 seats. "It will be harder than in the past", said Joan Botella, a political science professor at the Autonomous University of Barcelona (AUB).
Vox won 52 seats, up from the 24 seats with which it debuted in parliament in April. The country used to take pride in claiming that no far-right group had seats in the national Parliament, unlike the rest of Europe.
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Celebrating at the party's headquarters, where the crowd sang traditional Spanish songs and waved national flags, Cristina Martinez, a 25-year old lawyer, said: "Today Vox can not govern". But she said she cast her ballot in hopes of stopping Vox.
The 84-year-old did not want to say which party got his vote at a polling station in Madrid's northern Hortaleza neighbourhood where Abascal lives.
Vox relied on its anti-migrant message and attacks on laws that protect women from domestic abuse as well as what it considers leftist ideology disguised as political correctness. "PSOE will work to unblock the political situation", Sanchez told party supporters after the elections, adding that there would not be another snap general election in the country.
The most likely outcome appeared to be a minority Socialist government.
It has nevertheless flourished after recent riots in Catalonia by separatists, exploiting Spanish nationalist sentiment stirred up by the country's worst political conflict in decades.
The debate over Catalonia, meanwhile, promises to fester.
He repeated his pledge to end the Catalan crisis by suspending Catalonia's regional autonomy, banning separatist parties and arresting its regional president, Quim Torra, who has vowed to continue the secession drive.
Less than a month ago, Spain's Supreme Court sentenced nine Catalan separatist leaders to lengthy jail terms over their role in a failed 2017 independence bid, sparking days of angry street protests in Barcelona and other Catalan cities that sometimes turned violent.
More than 600 people were injured in the protests, which saw demonstrators torching barricades and throwing stones and Molotov cocktails at police.