Voters across Europe delivered a stinging rebuke to the political establishment in parliamentary elections Sunday, boosting formerly fringe parties to key wins and giving some of the European Union’s staunchest critics a prominent voice in the body’s future.

Parties from the center-left and the center-right managed to maintain their majority in the 751-member European Parliament and will continue to hold sway for the next five years. But the strong showing by anti-E.U. parties reflected widespread anger toward mainstream politicians and the appeal of populists at either end of the spectrum, particularly the far right.

Parties hostile toward the European Union were projected to come out on top in countries including Britain, Denmark, France and Greece. Continent-wide, they could be on pace to double their haul of parliamentary seats compared with the last election, in 2009.

“The people have spoken loud and clear,” a jubilant Marine Le Pen, leader of the far-right National Front, told cheering supporters after winning the French vote on a stridently anti-immigration message. “They no longer want to be led by those outside our borders, by E.U. commissioners and technocrats who are unelected. They want to be protected from globalization and take back the reins of their destiny.”

At stake in Sunday’s vote was the direction of Europe at a time when it is convalescing after four years of social and economic crisis.

Although the European Parliament has considerably less power than national governments in determining the direction of the 28-member union, its authority has been growing.

Among the issues on the agenda for the new Parliament will be the Transatlantic Trade and Investment Partnership, a trade deal between the United States and Europe that the Obama administration has made a priority of the president’s second term. Far-left and far-right parties in Europe have been highly critical of the proposed agreement.

The new Parliament also will have a say in electing Jose Manuel Barroso’s successor as president of the European Commission, the top job in the E.U.

Results released Sunday night suggested that the center-right faction — the European People’s Party — would win more seats than any other. That would put it in prime position to advance its candidate, Jean-Claude Juncker, a senior diplomat from Luxembourg and one of the European Union’s leading elder statesmen.

But for Juncker to get the job, the center-right is likely to need help from the center-left to overcome opposition on the extremes.

Despite the considerable stakes, the European campaign was marked by apathy, and turnout matched the record low set five years ago, at 43 percent.

Mainstream parties struggled to generate enthusiasm with their message of a more closely integrated union bound by common laws and open borders. Instead, the focus of the campaign was on the backlash and fears that integration has perhaps gone further than many Europeans want.

“European integration always seemed inevitable,” said Nigel Farage, leader of the U.K. Independence Party (UKIP), which came out on top in Britain’s vote. “And I think that inevitability will end tonight.”

That outcome could be welcome news for Russian President Vladimir Putin, whose nationalist style and promotion of traditional values have won him many fans on Europe’s far right.

Speaking via teleconference to an election-night gathering in Brussels, Farage blamed the European Union for provoking the crisis in Ukraine, saying that “if you poke the Russian bear with a stick, don’t be surprised when he reacts.”

UKIP’s win marked the first time in more than a century that a party other than Labor or Conservative has won a nationwide vote in Britain. To get there, UKIP rode a wave of anti-immigrant anger, as well as widespread disenchantment with the three establishment parties.

The charismatic and deeply controversial Farage had made victory in the European elections a top priority for his insurgent party, following a second-place finish five years ago.

Populism also found a strong perch in the economic ruins of Greece, where the leftist Syriza party staged a major victory. Meanwhile, the far-right Golden Dawn party — a group of extremists accused of harboring neo-Nazi beliefs and menacing immigrants — was clawing its way to a third-place finish that would send its members to the European Parliament for the first time.

The Greek vote gave a massive boost to Syriza’s charismatic leader, Alexis Tsipras, an anti-austerity crusader who is eying the Greek prime ministership and whose rising popularity has worried financial markets. The vote was considered an expression of rage in Greece, where six years of recession and high unemployment rates have soured many on the prescription of austerity and tough reforms to fix the nation’s ills.

Late Sunday, Tsipras said the vote showed that the center-right government had lost its legitimacy, and he called for snap elections.

But Greek Prime Minister Antonis Samaras remained defiant, insisting that the vote was not enough to force his government away from its path of reform.

In Italy, the elections were viewed as a referendum on the first three months in office of the new prime minister, Matteo Renzi, 39. In an important show of support, his Democratic Party was in first place, according to exit polls.

The anti-establishment Five Star Movement of comedian Beppe Grillo was running second. The movement’s showing suggested persistent discontent in Europe’s fourth-largest economy, where stagnant growth and a stubbornly high unemployment rate have fermented a backlash against austerity and reforms.

The night was not a total wash for those in favor of a more closely united Europe.

In Germany, Angela Merkel’s Christian Democratic Union and its sister party, the Christian Social Union, were projected to be the winners. Merkel has stood as a champion of a stronger, more integrated European Union, and the results in Germany illustrated her broad popularity in a country whose economy is the powerhouse of Europe and where job creation and economic growth have outpaced those of its neighbors.

Alternative für Deutschland, Germany’s relatively more moderate Euro-skeptic party, came in fourth, with 7 percent of the vote.

Nonetheless, the party said the momentum is on its side.

“It’s springtime in Germany,” leader Bernd Lucke told cheering supporters in Berlin. “Some flowers are blooming, and others are wilting.”

Faiola reported from Berlin. Stephanie Kirchner in Berlin contributed to this report.