BRUSSELS (Reuters) - The European Union's marathon parliamentary election kicked off on Thursday when polls opened in Britain and the Netherlands, where far-right, anti-EU parties are forecast to top the ballot, spearheading a surge in protest votes across the continent.
After two months of campaigning that opinion polls suggest has largely failed to inspire the electorate, up to 380 million Europeans are entitled to vote in 28 countries, choosing 751 deputies to represent them in the European Parliament.
Despite efforts to mobilize voters by telling them they will for the first time indirectly be choosing the next president of the European Commission, pollsters forecast a low turnout, possibly below the 2009 nadir of 43 percent.
With Europe struggling to recover from economic crisis, including record high unemployment and negligible growth, the election is expected to produce a surge in support for Eurosceptics on both the far-right and hard left.
In Britain, the UK Independence Party, which wants to withdraw from the EU and impose tighter immigration controls, is expected to win the vote, pushing the governing Conservatives into third place behind Labour, latest opinion polls show.
That could raise pressure on Conservative Prime Minister David Cameron, who has promised an in/out referendum on EU membership in 2017 if he is re-elected next year, to take a tougher line on reducing the EU's powers.
A similar story is expected in the Netherlands, where Geert Wilders' anti-Islam and anti-EU Freedom Party - which plans to forge an alliance with France's far-right National Front - is expected to win with up to 23 percent of the vote.
The Dutch will release exit polls on Thursday evening, but Britain will only announce its results late on Sunday, once voting has finished in all EU member states.
Consolidated results, including the allotment of seats in the parliament, will be announced at around 2100 GMT on Sunday.
The bulk of countries vote on May 25, when the trend towards the political extremes may become clearer, particularly in France, Denmark, Hungary, Italy, Greece and Austria.
On the last day of campaigning, Jean-Claude Juncker, the top candidate for Europe's centre-right political group, urged voters to steer away from the extremes.
"Do not give your votes to extremist xenophobes or fascists," the veteran former Luxembourg prime minister said at a rally in Brussels. "If you want Europe to function and to serve its citizens, we should vote for people who will work hard in the next European Parliament."
Juncker and his Socialist opponent, Martin Schulz, the German president of the outgoing European Parliament, have held an unprecedented series of television debates in an effort to personalize the election and enthuse the electorate.
Since the first direct elections to the European Parliament were held in 1979, turnout has fallen every time. It is expected to drop again to around 40 percent this year, pollsters say, a factor that will tend to boost the vote for radical parties.
That said, Europe's mainstream political groups - the centre-right European People's Party, the centre-left Socialists & Democrats, the liberal ALDE alliance and the Greens - are together expected to secure 70 percent of the vote, leaving them as a driving force in Europe as long as they work together.
In France, former President Nicolas Sarkozy, mostly absent from daily politics since being defeated in 2012, made a last-minute intervention in the campaign as his conservative UMP party risks being beaten into second place by Marine Le Pen's far-right National Front.
In an implicit swipe at his unpopular Socialist successor, Francois Hollande, Sarkozy called for a radical shake-up in the way the EU is run, with a Franco-German economic zone taking leadership of the euro zone at the centre of Europe.
He also called for the suspension of the EU's open-border Schengen zone of passport-free travel, which had failed to prevent an influx of migrants, and the negotiation of a stricter pact open only to countries with tougher immigration controls.
"We must stop believing in the myth of the equality of rights and responsibilities of all member states," Sarkozy wrote in an article in the French weekly magazine Le Point and the German daily Die Welt.
While the European Parliament has in the past been derided as a toothless talking shop, it has gained relevance since the passage of the Lisbon Treaty in 2009 and now enjoys 'co-decision' powers with member states over most legislation.
For the first time, parliament has also backed the idea that each group should have a "Spitzenkandidat" - German for "top candidate" - who is in line to become president of the European Commission should their group win the elections.
While supporters of the process are adamant it should be used to determine who succeeds Jose Manuel Barroso as Commission president, one of Brussels' most influential jobs, EU leaders are ultimately responsible for putting forward a name.
According to the Lisbon Treaty, they must "take into account" the election results in making the nomination, and that person must then be approved by a majority in parliament.
If EU leaders and parliament cannot agree on the candidate, there is the risk of an institutional impasse in Brussels, which could have long-term repercussions on confidence in the EU among already disillusioned voters and financial markets.
Parliamentary leaders will meet on the morning of May 27 to assess the outcome of the elections, and EU heads of state and government will do the same over dinner the same day. But there is not expected to be any clarity on the nominee for Commission president until later in June, EU officials say.
While the main Eurosceptic assault in many countries comes from the far right, the main challenge to new Italian Prime Minister Matteo Renzi's centre-left Democratic Party comes from the anti-establishment 5-Star Movement of former comic Beppe Grillo.
The last surveys released before a blackout on publishing opinion polls gave the Democrats a comfortable lead, but private polls leaked since then suggest it may be a tighter race.
Additional reporting by John O'Donnell and Julia Fioretti; Writing by Luke Baker and Paul Taylor; Editing by Will Waterman