LONDON, England (CNN) -- Voters in the United Kingdom and the Netherlands became the first European Union citizens to go to the polls Thursday to elect a new European Parliament.
A voting clerk awaits voters outside a voiting station in Salisbury, southern England.
The massive election, involving all 27 member states, around 375 million eligible voters and 736 MEP seats, is the biggest exercise in transnational democracy the world has ever seen, with voters from Bulgaria and Romania, which joined the EU in 2007, participating for the first time.
Each country in the EU sets its own timetable for voting with most going to the polls on Sunday. In Ireland and the Czech Republic voting takes place on Friday. Counting is due to start on Sunday night.
The powers of the European Parliament -- the EU's main legislative assembly -- have increased significantly since it was established in 1979, with the parliament often shaping legislation which is then passed down to national parliaments for ratification at member state level.
But there have been fears that this year's contest could be hijacked by extremist parties taking advantage of low turnout across member states.
A "Euro barometer" poll conducted last month found that only a third of potential voters intended to use their mandate, suggesting that participation could be even lower than the 45 percent turnout in 2004.
In a statement issued Thursday, European Parliament President Hans-Gert Pottering urged all citizens to use their vote to "support those parties that are in favour of European integration and that are committed to our European values."
"The elections to the European Parliament are of a greatest importance. They are a great occasion to re-launch the European Union and to address the issues that are important to citizens such as the economic crisis, unemployment, climate change, energy security, globalization or immigration," Pottering said.
In the UK, voting is expected to be dominated by dissatisfaction with beleaguered Prime Minister Gordon Brown's Labour government, as well as a backlash against mainstream parties in the wake of the recent MPs expenses scandal.
That could benefit marginal parties, raising concerns that the far right anti-immigrant British National Party could win enough votes to send its first MEP to Brussels. The Green Party and the anti-Europe UK Independence Party have also been tipped to increase their share of the vote.
In Germany, European elections are being treated as a rehearsal ahead of a general election later this year with Chancellor Angela Merkel struggling to hold together her "Grand Coalition" with the Social Democrats and the upwardly-mobile and anti-capitalist Left Party expected to capitalize on resentment with the two main parties.
In France, the opposition Socialists are encouraging voters to treat the European election as a poll on President Sarkozy's record so far in the Elysee Palace.
The European results will be seen as a test for the new Socialist leader Martine Aubry. But with the French tradition of street politics and the continuing appeal of radical traditions, there will be much interest in the performance of the far left New Anti-Capitalist Party led by Marxist postman Olivier Besancenot.
In Italy Premier Silvio Berlusconi has formed yet another new party coalition -- People of Freedom (PdL) -- merging his Forza Italia with the post-fascist Allianza Nationale of Gianfranco Fini. Immigration and the controversy over the treatment of its Roma population is likely to figure in the campaign.
A particularly tight contest seems likely in Poland where polls show the governing center-right (PO) party of Prime Minister Donald Tusk running level with the conservative Law and Justice (PiS). Electors are not much interested though with only 17 percent of Poles saying they are likely to vote.