Eurosceptic and far-right parties have seized ground in elections to the European parliament, in what France's PM called a "political earthquake".
While the French National Front and UK Independence Party both appear headed for first place, the three big centrist blocs in parliament all lost seats.
The outcome means a greater say for those who want to cut back the EU's powers, or abolish it completely.
But EU supporters will be pleased that election turnout was slightly higher.
It was 43.1%, according to provisional European Parliament figures. That would be the first time turnout had not fallen since the previous election - but would only be an improvement of 0.1%.
"The people have spoken loud and clear," a triumphant Marine Le Pen told cheering supporters at National Front (FN) party headquarters in 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 globalisation and take back the reins of their destiny."
Provisional results suggested the FN could win 25 European Parliament seats - a stunning increase on its three in 2009.
The party also issued an extraordinary statement accusing the government of vote-rigging.
Across the board, the centre-right European People's Party was set to win 211 out of the 751 seats, with 28.1% across the bloc, according to estimated results issued by the European Parliament. That would make it the biggest group - but with more than 60 seats fewer than before.
That put it ahead of the Socialist group with 193 seats (25.7%), Liberals with 74 (9.9%) and Greens 58 (7.7%).
The right-wing Eurosceptic Europe of Freedom and Democracy group, incorporating the UK Independence Party (UKIP), looked to have about the same number of seats as last time, but there looked set to be a rise in the number of non-attached right-wing MEPs boosting the Eurosceptic camp.
UKIP leader Nigel Farage was predicting that his party would come first in Britain, saying: "The inevitability of European integration ends tonight."
The anti-bailout hard-left group in parliament was set to make big gains, largely thanks to Syriza in Greece and United Left in Spain, gaining about 12 seats.
The election is the biggest exercise in multi-national democracy in the world. The vote will affect the lives of the EU's 500 million citizens.
The parliament's powers have expanded since the last election in 2009, and it is hoping to have a decisive say in who gets the EU's top job, president of the European Commission.
The frontrunner for that will now be EPP candidate Jean-Claude Juncker - though that would need the approval national government leaders, which is by no means guaranteed, correspondents say.