Madrid, Nov 11 (AFP) Voting drew to a close in Spain's fourth general election in as many years on Sunday, with the ballot overshadowed by the ongoing Catalan separatist crisis which has fuelled support for the upstart far-right party Vox.

As polling stations closed at 1900 GMT, figures from several opinion polls carried out in recent days but which could only be published after voting ended, suggested Prime Minister Pedro Sanchez's Socialists would win with nearly 120 seats.

In the last election in April, the Socialists secured 123 of the parliament's 350 seats, falling far short of an absolute majority, and unable to form a government Sanchez was forced to call a new vote.

The survey also predicted a bigger-than-expected surge for Vox with around 50 seats, easily doubling the 24 it won in April when it made its parliamentary debut in the biggest showing by the far-right since Spain returned to democracy after dictator Francisco Franco's death in 1975.

Such a result would make Vox the third-largest party in parliament, after the rightwing Popular Party, which was seen taking just under 90 seats, up from 66 in April.

In recent days, Sanchez has repeatedly raised the alarm about Vox's "aggressive ultra-rightwing" policies, warning the party would drag the country back to the dark days of Franco's dictatorship.

The last election produced a near-record 76 percent turnout, which helped Sanchez who had mobilised left-leaning voters to oppose Vox.

But by 6:00 pm (1700 GMT) turnout stood at 56.9 percent, nearly four percentage points lower than at the same hour in the April ballot.

Spain does not publish an exit poll, with preliminary results expected in a few hours.

The campaign took place on the heels of a fresh wave of demonstrations in Catalonia.

Since mid-October when Spain's Supreme Court sentenced nine Catalan leaders to lengthy jail terms over their role in a failed 2017 independence bid, separatist protesters have staged mass protests.

But by night, the protests initially descended into violent clashes, with masked demonstrators torching barricades and hurling rocks and Molotov cocktails at riot police who hit back with water canon, tear gas and foam bullets.

More than 600 people were injured, around half of them police.

As the crisis gathered pace, Sanchez came under increasing pressure from the right to clamp down on the unrest, with Vox leader Santiago Abascal calling for Madrid to suspend Catalonia's autonomy and arrest regional president Quim Torra.

At his final campaign rally on Friday, Abascal -- who has pledged to ban all separatist parties -- said "drastic solutions" were needed as his supporters chanted: "Torra to the dungeon!"

With his rallying cry of "Spaniards first" and his untempered rage against those who would betray the unity of Spain, Abascal has capitalised on the crisis, leading to a surge in support for his far-right faction.

Launched in 2014, Vox initially struggled to gain traction with its ultra-conservative stance on immigrants, gender violence and traditional values, but over the past year, it has chalked up significant gains Lidia Lopez, a 21-year-old trainee journalist who lives in the same Madrid neighbourhood as Abascal, said the extremist leader had benefitted from the spotlight offered by televised election debates.

"But the problem is that during such debates, he uses falsehoods and nobody checks in real-time to see if what he's saying is true or not, so people hear it and believe it is true," she said after casting her ballot for the radical left Podemos.

The party has repeatedly come under fire over false claims in its campaign, with more than 2,500 academics and researchers on Friday denouncing Vox for its "calculated, systemic and recurrent" use of "falsified and manipulated data".

Spain has been caught in political paralysis since the election of December 2015 when Podemos and business-friendly Ciudadanos entered parliament.

That put an end to decades of dominance of the two main parties, the PP and the Socialists, in the eurozone's fourth-largest economy.

There is a good risk Sunday's vote will only prolong the agony.

With no single party able to secure the required 176 seats for a majority, the Socialists are likely to opt for a minority government, ING analyst Steven Trypsteen said.

"Voting intentions appear to have changed since the April election. But these changes will not make it easier to form a government," he added. (AFP)

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