By Noah Barkin

VIENNA (Reuters) - Cheered on by thousands of flag-waving Austrians, the leaders of Europe's biggest far-right parties railed on Friday against the European Union and Islam and urged Britons to free themselves from what they called heartless EU technocrats.

The rally on the outskirts of Vienna brought together an array of anti-immigration, anti-EU parties that have unsettled a European political establishment still struggling to get a grip on a historic refugee crisis and years of economic weakness.

The parties had earlier pledged more cooperation at a meeting hosted by Heinz-Christian Strache, whose Freedom Party (FPO) came within a whisker of winning the Austrian presidency last month and is now challenging the result.

Attendees included Marine Le Pen, the leader of France's National Front, and politicians from the Alternative for Germany (AfD) and Northern League of Italy.

"The fear-mongering of people like (Jean-Claude) Juncker and (Martin) Schulz cannot sway us," Le Pen told a crowd of about 2,000 at the pyramid-shaped convention center, referring to the heads of the European Commission and European Parliament.

"They are worried that Britain might win back its freedom," she added, to cheers. "We want Britons to set themselves free."

Speaking after her, Strache accused German Chancellor Angela Merkel of causing "irreparable damage" to Europe by opening German borders to hundreds of thousands of refugees fleeing war in the Middle East.

"We are not against Europe as our opponents are always saying. We want another Europe, a better Europe, one of nations, values, culture and identity," Strache said. "The new fascism comes from the left and from radical Islam."


Held under the slogan "Patriotic Spring -- Cooperation for Peace, Security and Prosperity in Europe", the gathering aimed to strengthen ties between like-minded parties whose nationalist tendencies have hampered close collaboration in the past.

At a morning news conference in the Austrian parliament Strache and Le Pen expressed hope that Britain's June 23 vote on whether to remain a member of the European Union would give their cause new momentum.

"I support the referendum in the United Kingdom because I want all the countries in the EU to have this choice," Le Pen said. "But even if we don't get Brexit, it will present a huge new problem for the European Union which has pledged to give Britain special rights if it stays that other countries won't have. So this could be the beginning of Europe a la carte."

Populist, anti-immigration parties are on the rise across Europe as high unemployment and austerity, the arrival of record numbers of refugees, and recent militant attacks in France and Belgium deepen voter disillusionment with traditional parties.

The mood is mirrored in the United States, where Donald Trump has confounded the political establishment by crushing rivals for the Republican presidential nomination with rhetoric that has been widely denounced as racist and divisive.

Le Pen is expected to make it into a second-round run-off for the French presidency next year.

In neighboring Germany, where far-right parties have struggled to gain traction in the post-war era, the AfD has won double-digit support in a string of state elections and seems poised to enter the Bundestag in Berlin next year.

AfD leader Frauke Petry joined Strache last week for a symbolic trip to the top of the Zugspitze, Germany's highest mountain, and her partner, AfD politician Marcus Pretzell, joined the gathering in Vienna.

"Patriots love what Germany once was, what Germany could be. But they cry when they look at the current state of their country," Pretzell told the crowd.

(Editing by Catherine Evans)