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Without waiting for the final result, a sombre French Prime Minister Manuel Valls went on television to call the breakthrough by the anti-immigration, anti-euro party in one of the EU's founding nations "an earthquake" for France and Europe.
Far right and hard left parties, their scores magnified by a low turnout of 43.1 percent, gained ground in many countries although in Germany, the EU's biggest member state with the largest number of seats, the pro-European centre ground held firm, according to exit polls.
A jubilant Le Pen, whose party beat President Francois Hollande's ruling Socialists into third place, told supporters: "The people have spoken loud and clear... they no longer want to be led by those outside our borders, by EU commissioners and technocrats who are unelected.
"They want to be protected from globalisation and take back the reins of their destiny."
The National Front was set to win more than 25 percent of the vote, comfortably ahead of the conservative opposition UMP on 21 percent, with the Socialists on 14.5 percent, their second defeat in two months after losing dozens of town halls in March.
With first official results from around the 28-nation bloc due late on Sunday evening, pro-European centre-left and centre-right parties seemed sure to maintain control of the 751-seat EU legislature, but the number of Eurosceptic members may double.
The centre-right European People's Party, led by former Luxembourg Prime Minister Jean-Claude Juncker, was on track to win 211 seats, early projections of the vote by the parliament showed.
The centre-left Socialists led by outgoing European Parliament President Martin Schulz of Germany were seen in second place with 193 seats.
The political fallout may be felt more strongly in national politics than at EU level, pulling mainstream conservative parties further to the right and raising pressure to crack down on immigration.
Denmark's anti-immigration far right People's Party was set to top the poll with an estimated 23 percent and the extreme-right Jobbik, widely accused of racism and anti-Semitism, was running second in Hungary with 15 percent.
The UK Independence Party, which campaigns for Britain to leave the EU, was set for a strong score after making big gains in local elections held at the same time on Thursday, raising pressure on Conservative Prime Minister David Cameron ahead of a general election next year.
In one apparent exception to the trend, the anti-Islam, Eurosceptic Dutch Freedom Party of Geert Wilders' - which plans to forge an alliance with Le Pen - fell well short of its goal of topping the poll.
Although 388 million Europeans were eligible to vote, average turnout was officially estimated at 43.1 percent, barely higher than the 2009 nadir of 43.0 percent, despite efforts to personalise the election with all the mainstream political families putting forward a leading candidate or "Spitzenkandidat".
In Germany, Chancellor Angela Merkel's Christian Democrats were set to 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 centre-left Social Democrats, her coalition partners, were forecast to take 27.5 percent.
The anti-euro Alternative for Germany (AfD) 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, epicentre of the euro zone's debt crisis, where the radical left anti-austerity Syriza movement of Alexis Tsipras was set to win with 26.7 percent, pushing governing New Democracy into second place on 22.8 percent.
That reflected popular anger at harsh spending cuts the government has adopted in recent years to meet the terms of its EU/IMF bailout programme.
The surge in support for the far left raises doubts about how much longer the coalition government can last with a parliamentary majority of just two seats, although government spokesman Simos Kedikoglou said it would endure.
"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."
The two parties in the coalition, New Democracy and PASOK, won a combined vote larger than that of Syriza.
Sunday was the fourth and final day of voting in elections to 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, enough to gain a much louder voice but probably not to block EU legislation.
Officials said final results and seat allotments would likely not be finalised until later on Monday.
The record low turnout was in Slovakia, with just 13 percent. The highest was 90 percent in Belgium, where voting is compulsory and there was a general election on the same day.
Sweden appeared to have elected the only feminist party member of the EU assembly.