Critics of the European Union, riding a wave of anger over austerity and mass unemployment, gained ground elsewhere but in Germany, the EU's biggest member state, the pro-European center ground held firm, according to exit polls.
In France, Marine Le Pen's nationalist movement which blames Brussels for everything from immigration to job losses, was set to take about 25 percent of the vote, comfortably ahead of the conservative opposition UMP on about 21 percent.
President Francois Hollande's Socialists suffered their second electoral humiliation in two months after losing dozens of town halls, trailing far behind in third place with about 14.5 percent, according to projections based on partial results.
With officials results from around the 28-nation bloc due late on Sunday evening, pro-European center-left and center-right parties seemed sure to maintain control of the 751-seat EU legislature, but the number of Eurosceptic members may double.
In Britain, the UK Independence Party, which campaigns to leave the European Union, was set for a strong score after making big gains in local elections held at the same time on Thursday.
But in the Netherlands, the anti-Islam, Eurosceptic Freedom Party of Geert Wilders' - which plans to forge an alliance with France's National Front - fell well short of its goal of topping the poll.
Projections by German television indicated that Angela Merkel's Christian Democrats would secure 36 percent of the vote, down from a 23-year-high of 41.5 percent in last year's federal election but still a clear victory.
The center-left Social Democrats were forecast to take 27.5 percent, according to public broadcaster ARD, with turnout up from the last European elections in 2009.
The anti-euro Alternative for Germany (AfD) party won parliamentary representation for the first time with an estimated 6.5 percent, the best result so far for a conservative
party created only last year.
"Germany has cast a clear pro-Europe vote and the high turnout is a good signal for the idea of European unity," said David McAllister, the top Christian Democrat candidate.
GREEK FAR LEFT GAINS
It was a different story in Greece, epicenter of the euro zone's debt crisis, where the far-left anti-austerity Syriza movement of Alexis Tsipras was expected to take 26-30 percent of the vote, pushing governing New Democracy into second place.
That would appear to reflect popular frustration with the harsh spending cuts the government has adopted in recent years to meet the terms of its economic rescue program.
The surge in support for the far-left raises doubts about how much longer the center-right government can last with a parliamentary majority of just two seats, although government spokesman Simos Kedikoglou said there was no question that the government would not finish its four-year term.
"It's easy for people to cast a protest vote in European elections," he told Greek television. "The political scenario of a government collapse, which Syriza was trying to paint, has not been borne out by the facts."
Sunday was the fourth and final day of voting in the European elections, with up to 388 million Europeans eligible to vote for their representatives in the European Parliament, which is an equal co-legislator with member states on most EU laws.
Far-right and radical left groups were expected to secure up to a quarter of the seats in parliament, enough to gain a much louder voice but probably not to block EU legislation.
The European Parliament will announce first results after 2100 GMT on Sunday, once polls have closed in Italy. But officials said final results and seat allotments would likely not be finalized until later on Monday.
Turnout - the most basic measure of citizens' engagement with Europe - had been expected to fall from the historic low of 43 percent registered in 2009, but indications from Germany and Sweden were that it was higher than expected.
At the same time, it was forecast to hit record lows in Slovakia, the Czech Republic and Britain, highlighting the ambivalence citizens in many countries feel towards Europe.