Exit polls across the European Union show surprisingly strong wins for so-called Eurosceptics on the far right and far left in European Parliament elections.
French Prime Minister Manuel Valls called Sunday's results "an earthquake" for Europe and France, where Marine Le Pen's far right National Front party was the outright winner. National Front will have seats in the parliament for the first time.
Nationalist parties are also expected to win in Denmark and Britain.
In Greece, the radical leftist coalition is poised to win. Greek leftists strongly oppose the government's economic austerity and the terms of the EU bailout for Greece.
The far right parties are known for opposing immigration and generally want to slash the powers of the European Union.
Official results of the European Parliament elections are expected Monday.
Voters in 21 of the European Union's 28 nations voted Sunday. The other seven countries had already voted.
“The people have spoken loud and clear,” a triumphant Le Pen told cheering supporters at party headquarters in a northwest suburb of Paris.
“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 globalization and take back the reins of their destiny,” she said.
TV exit polls all gave the FN around 25 percent of the vote, a few points higher than expected in pre-vote surveys.
The mainstream UMP conservative opposition was put on 21 percent. Hollande's Socialists stood at between 14 and 14.5 percent - well down from the 16.5 percent they won last time in 2009 and close to a record low score achieved in 1994.
The FN, which Le Pen has sought to rid of its racist reputation since she took the reins from her father in 2011, campaigned on a platform of tighter borders, hostility to the euro currency and rejection of a planned EU-US trade pact.
Searching for allies
As expected in the event of an FN victory, Le Pen called for the dissolution of France's national assembly, insisting it was no longer representative of the French people's will. The FN holds only two of 577 seats in the national parliament.
Ruling Socialists quickly dismissed such a move which, given Hollande's record unpopularity, would almost certainly mean that his allies would lose their current majority in parliament.
If confirmed by preliminary official results due later on Sunday, the score would also be a slap for the conservative UMP, whose chairman Jean-Francois Cope is expected to face challenges for the leadership of the party.
“It's a big disappointment for us, but mostly an expression of anger from the French people,” Cope said.
Survey group Ifop said the abstention rate was 59 percent slightly lower than the record high many pollsters had expected.
The FN has carved out a niche for itself in French politics, largely among disaffected working class voters who believe the mainstream elite does not understand their concerns about crime, immigration and joblessness.
In March it won control of a record 11 town halls across France - a tiny minority of France's thousands of municipalities but still a slap in the face for the mainstream left and right which have always ruled France.
Le Pen told Reuters in the run-up to the vote that a major priority for the FN and other Eurosceptic parties across Europe would be to form a political caucus in the European Parliament and aim to block a planned transatlantic trade pact.
Pollster Ifop estimated the FN would secure between 22 and 25 seats in the EU assembly.
Information for this report was contributed by Reuters.