Voters across the European Union on Sunday cast their ballots in the final day of European parliamentary elections, a huge democratic exercise in which far-right populists and eurosceptics are set to make gains amid a downward trend in support for mainstream parties.

With voting already completed in the United Kingdom, the Netherlands and the Republic of Ireland, tens of millions of eligible voters in 21 countries, including heavyweights like Germany, France, Italy, and Spain, are called to polling stations on the final day of the process to elect members of the EU's legislative branch.

In Germany, which as the most populous EU nation elects 96 MEPs to the 751-seat chamber, around 64.9 million people are entitled to vote in the parliamentary elections, which are being held simultaneously with regional and local elections.

Polling during the campaign suggested Chancellor Angela Merkel's conservative bloc, whose candidate for European Commission chief is Manfred Weber of the European People's Party (EPP), would maintain its leading position in the election but would shed votes compared to its performance five years ago.

The latest poll, published by public TV channel ZDF, gave 28 percent of the vote to Merkel's Christian Democratic Union/Christian Social Union in Bavaria (CSU) bloc, followed by the Greens, with 18.5 percent.

The Social Democratic Party of Germany (SPD), was on track to drop by 10 percent while the far-right Alternative For Germany (AfD), would cement itself as the German fourth power with 12 percent of the tally, up from 7 percent in 2014.

Across the border in France, another EU powerhouse, French President Emmanuel Macron's pro-European and centrist vision is coming up against the Euroscepticism of Marine Le Pen's far-right National Rally (RN), formerly known as the National Front (FN).

Pollsters had the pair at a knife-edge in the lead up to election day with some indicating that Le Pen's hardline anti-EU party could take a larger share than Macron's Republic on the Move (LREM) and its centrist allies.

In Italy, the European elections are acting as a referendum on the popularity of the Matteo Salvini's far-right League party and the anti-establishment Five Star Movement (M5S), which collaborate as coalition partners in Italy's national government.

Salvini has been a driving force in trying to unite European right-wing parties in an attempt to consolidate the anti-immigration, eurosceptic voice in the EU chamber and could come out on top in the Italian vote if polling is accurate, a position formerly held by M5S, whose leader is Luigi di Maio.

Italy's prime minister, Giuseppe Conte, a figure agreed on by both coalition parties, has urged for deep reform in the EU.

In Hungary, the nationalist Fidesz party of Prime Minister Viktor Orbán is predicted to sweep up the election with as much as 52 percent of the Hungarian vote.

One question hanging over the country, however, is whether Fidesz will keep its 13 or 14 EU seats with the EPP or send them to join the far-right coalition with Salvini.

Orbán's hard-line politics have kept his group on the fringe of the EPP.

Spain, like Germany, was holding its EU election voting process in tangent with municipal and regional elections.

Here, too, the vote is regarded as test for acting Prime Minister Pedro Sánchez's Socialist Party (PSOE), to see whether the center-left group can extend recent achievements in a snap general election, in which it became the biggest party amid a collapse in support for the Popular Party, a powerhouse of Spanish conservatism.

Austria's political landscape has been tumultuous in recent days following the collapse of the coalition between the conservative Austrian People's Party (ÖVP) of Chancellor Sebastian Kurz, and the far-right Freedom Party (FPÖ).

The shock came after the FPÖ's former leader was caught on camera suggesting he was prepared to accept illegal money in exchange for handing out favorable public contracts to a woman who was posing as the daughter of a Russian oligarch during a private meeting in Ibiza in 2017.

Ibizagate, as it has come to be known, could hand a boost to the Austrian opposition, specifically the Social Democratic Party.

Voting was also underway in the bloc's newer members.

In Romania, much of the debate was centered on corruption, a topic that has plagued the country's politics for years.

In fact, the voting process is to be held alongside a referendum on recent government reforms to the justice system.

The European Commission, President Klaus Iohannis and the opposition have all accused the Social Democratic Party (PSD)-led government of halting progress in the fight against corruption after it approved reforms that reduced sentences and the length of statutes of limitations for corruption crimes.

PSD leader Liviu Dragnea already has a suspended sentence for such misdemeanors.

Other countries casting votes on the final day of the elections include Portugal, Luxembourg, Denmark, Sweden, Finland, Estonia, Lithuania, Poland, Slovenia, Croatia, Bulgaria, Greece, Belgium and Cyprus.

Full results are not expected to be revealed until the early hours of Monday. EFE-EPA