Several French polling institutes estimated the final turnout could be over 50 per cent at the end of the day, which would be the highest in the country since 1994.
Macron's pro-EU centrists will join a bloc of liberals - who after tonight's success are set to be the third largest group in the Parliament - that will soon wield enough power to dictate its vision of Europe to others.
The Brexit process, or the Article 50 procedures that began over two years ago, should have seen Britain leave the European Union in time to sit out the elections, but an unplanned extension of the process - until October 31, 2019, at least under the current Brexit schedule - has left Britain obliged to participate in the election cycle.
Germany's Manfred Weber, the candidate of the European People's Party, now the biggest in the legislature, said in Berlin that the elections appeared to have weakened the political center.
It was a patchy night overall for the populists and right-wing nationalists, but they topped the polls in France, Italy and the UK.
In Germany, the biggest member state and one of 21 countries that voted on Sunday, an exit poll for public broadcaster ARD showed the Greens on 22 per cent, Merkel's Christian Democrats (CDU) and their conservative allies on 28 per cent, down eight points from 2014, and the Social Democrats slumping almost 12 points to 15.5 per cent.
Five years ago, more than 80% of voters cast their votes in Belgium and Luxembourg, where voting is obligatory, while only 13% took part in the election in the ex-communist Slovakia, according to the European Parliament's figures.
About 350 million people throughout the EU countries were eligible to vote to elect 751 members of the European Parliament, who regularly pass laws affecting the bloc. The 5-Star movement, led by another deputy PM, Luigi Di Maio, took between 18.5 and 23 percent, likely getting overtaken by the center-left Democratic Party, which is expected to receive some 20.5-25 percent of the votes.
The far-right Alternative for Germany, though, was set for an indifferent performance.
The balloting, which began Thursday, pitted supporters of closer unity against those who consider the European Union a meddlesome and bureaucratic presence and want to return power to national governments and sharply restrict immigration.
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But if those polls bear out, the gains made by the populists and nationalists - combined with a strong performance of green parties - appear to have continued the weakening of Europe's traditional mainstream parties.
By late Sunday, exit polls in Germany and Greece showed that centrist governing parties would lose seats in the new formation of the continent-wide parliament.
Romanians overseas also seemed to have turned out for the referendum in large numbers, with long queues of voters reported at Romanian diplomatic missions in Paris, London, Munich and Milan, some of them posting vehemently anti-PSD slogans on social media.
Speaking at the European election count centre in Dublin, Mr Varadkar said there are a number of factors at play including Brexit.
That's bad news not just for Macron - but also for the French leader's grand ambitions for a more united Europe.
Hungary's increasingly authoritarian prime minister Viktor Orban, a possible ally of Salvini, said he hopes the election will bring a shift toward political parties that want to stop migration.
Voters from the EU's 28 member states head to the polls this week for the ninth time since 1979, to elect their representatives to the European Parliament.
People pick ballot papers at a polling station in Barcelona, Spain, May 26, 2019, in elections for the European Union parliament.
Voters in Brexit-bound Britain cast ballots Thursday at the start of 28-nation European Union elections in which Eurosceptic, anti-immigration forces have vowed a political quake to shake up the Brussels establishment.