In Greece, with a quarter of the votes counted, the extreme right Golden Dawn party was third with 9.33 percent.
Doru Frantescu, policy director of VoteWatch Europe, an independent Brussels-based organization, said Europe's mainstream political parties won enough seats to still muster a majority on issues where they concur.
"The problem comes when the left, the Socialists and EPP will not agree on issues," Frantescu said.
In the incoming European Parliament, he said, fringe parties will be able to exert more pressure on key topics, ranging from how liberal to make the internal European market for services or the proper mix of energy sources to which clauses should be scrapped in a proposed trade and investment agreement with the U.S.
In the Netherlands, however, the right-wing Euroskeptic Party for Freedom surprisingly dropped a seat from five to four. Its outspoken leader, Geert Wilders, said in a statement his party looked forward to working with Le Pen in Europe, calling the National Front leader "the next French president."
In Italy, early projections indicated that the main government party, the Democrats led by Premier Matteo Renzi, had beaten off a challenge by the anti-euro 5-Star Movement of comic Beppe Grillo. The center-left Democrats were forecast to win 40 percent, while Grillo's anti-establishment movement would garner 22.5 percent.
Despite the gains, unity may be hard to find in the fractured Euroskeptic camp.
Le Pen has said she will work with Wilders' party but Britain's Farage has ruled out cooperating with both those parties, which have stridently anti-immigrant platforms.
"We won't work with right-wing populists," Alternative for Germany's leader Bernd Lucke also said after the vote, insisting his party was generally in favor of the EU despite its rejection of the common currency.
Grillo in the past has said his movement wouldn't ally itself with Le Pen's party, claiming the 5-Stars have a different "DNA."
Conservative caucus leader Joseph Daul put a brave face on the results Sunday.
"One thing remains certain: EPP is the responsible political force in Europe, which keeps Europe open," he said.
The European Parliament estimated turnout was narrowly up from the last election in 2009, at 43.1 percent, reversing years of declining turnouts.
Voters also put new parties in the European Parliament, with preliminary results showing that Sweden elected the first lawmaker from a feminist party and the Dutch returned one representative for the Party for the Animals.
"You know that we have created history don't you? We inspire the world. This is the force of love!" the Feminist Initiative's main candidate, Soraya Post, proclaimed in front of cheering supporters in Stockholm.
AP correspondents John-Thor Dahlburg in Brussels, Jill Lawless in London, Elaine Ganley in Paris, Frank Jordans in Berlin and Ciaran Giles in Madrid contributed.