Marine Le Pen, president of the Front National (FN), delivers a speech in Nanterre, near Paris, after the far-right FN was the top vote winner in France's election for the European Parliament on May 25. (Yoan Valat/EPA)

From France and Britain to Denmark and Greece, voters may have just put the brakes on the Golden Age of European integration.

Stunned leaders across the region were grappling Monday with the aftershocks of Sunday’s vote for the 751-member European Parliament that saw a record number of seats go to once-fringe parties bitterly opposed to a united Europe. The results presented the region’s mainstream parties with a reality check: Maybe Europeans aren’t quite as eager as their leaders think they are to forge a joint future.

In the decades after World War II, nations across the region embraced a common path, boldly moving to open borders, create a common currency and cede ­national powers to a European Union authority based in the region’s administrative capital of Brussels. With little more than a plane ticket and a plan, Polish plumbers could find work in Paris or London; Italian engineers could labor inside German auto factories; Greek cooks could set up shop in Barcelona, Copenhagen or Amsterdam.

But after four years of economic devastation from Europe’s debt crisis, the love-thy-neighbor shtick is wearing thin for millions of European voters. Even pillar countries such as France, now mired in economic stagnation and hopelessness, are increasingly blaming the E.U. for their woes.

At the same time, the results suggested a strong backlash to German-led demands for fiscal austerity and economic reforms that voters across the region are blaming for massive unemployment, particularly in troubled Greece.

Indeed, by voting to send a sea of nationalists and extremists from the far right and left to the region’s centerpiece legislature, voters have effectively put the foxes in the henhouse, giving a new platform to those who want to turn back the clock on European integration.

At the very least, analysts say, the vote could give new momentum to a push being led by British Prime Minister David Cameron to reclaim some powers ceded by national governments to Brussels, effectively reversing the trend of the past several decades toward integration.

The outcome of the elections was “a massive rejection of the European Union,” Marine Le Pen , whose anti-E.U. French nationalists came in first Sunday in their best showing ever, told supporters after the results were released. She added that the French, at least, “don’t want to be led from the outside anymore.”

Although anti-E.U. parties staged their strongest showing ever, they are still on track to secure less than a third of the seats in the European Parliament — not enough to create gridlock. In fact, analysts say one byproduct of the vote could be a move by pro-E.U. parties from the center-right and center-left to work ­together more in Parliament to promote the integration agenda.

Yet the surging fringe groups could pose a danger to certain key pieces of legislation — particularly the free-trade agreement being negotiated between the E.U. and the United States.

“If you bring in a big bloc like this, the risk is they are able to sometimes bring with it more mainstream parliamentarians to stop certain issues, and that could certainly apply to free trade,” said Mats Persson, director of the London-based think tank Open Europe. “These groups tend to be very protectionist.”

The vote also could put fresh wind in the sails of Cameron’s push to water down the E.U.

He has vowed to give British voters a chance to get out of the E.U. by the end of 2017 if he is reelected next May. Before then, he is calling for renegotiated rules of membership, which he calls essential if Britain is to remain part of the union. Those rules could involve new restrictions on freedom of movement and trade laws, some of which are opposed by pro-European leaders on the continent.

But the rise of extreme parties will likely be felt first and foremost in their home countries, where mainstream parties such as Cameron’s Conservatives have found themselves losing a substantial number of voters to the fringe parties now on the rise.

In Britain’s case, the anti-
immigration United Kingdom Independence Party emerged on top in last week’s balloting. While UKIP is likely to win at best a handful of seats in next year’s national elections, the party could play a spoiler role that might undo the Conservatives and lift the opposition Labor Party back to power.

At the same time, the Liberal Democrats — the only major party in Britain to campaign on a pro-European line — lost 10 of its 11 seats in the European Parliament. Its disastrous performance revealed just how politically untenable such positions have become at a time when voters are furious at the E.U. for lax immigration rules that have allowed massive waves of eastern and southern Europeans to enter Britain in the past decade.

There also are signs that the strength of the far right in particular is forcing even mainstream politicians to adopt more extreme positions to better appeal to voters. Even staunchly pro-European politicians such as German Chancellor Angela Merkel are adopting harder stances, with Berlin insisting, for instance, that more must be done to stop so-called “benefit tourism” — the practice of E.U. citizens from weaker countries such as Bulgaria and Romania relocating to countries such as Germany to take advantage of their generous ­welfare systems.

On Monday, however, Merkel called the rise of right-wing populist parties in the European elections “remarkable and regrettable.” She also ruled out a collaboration with the E.U.-skeptic Alternative für Deutschland (AfD) party, which won 7 percent of the vote in Germany.

In France, meanwhile, the unpopular Socialist government of President François Hollande was making fresh vows Monday to win back the confidence of the nation. Prime Minister Manuel Valls immediately promised a new round of tax cuts.

“There must be new tax cuts, because the tax burden has become unbearable,” Valls said Monday in an interview with French radio. “There has been considerable skepticism towards those who govern,” the prime minister conceded, but the government has to face up to its responsibility to govern and not let the extreme right take over, Valls added.

Witte reported from London. Stephanie Kirchner in Berlin contributed to this report.