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From Paris to Warsaw to Berlin, Europeans have reacted to the British referendum with a resounding message: “Please don’t go!”

Those words recently appeared on the cover of Der Spiegel, an influential German magazine, which made an impassioned plea to Britons not to vote to leave the 28-member bloc.

“Were the British to leave the E.U., it would be a threefold catastrophe: bad for Germany, bad for Britain and cataclysmic for Europe,” said an appeal published this month in English on the magazine’s website.

Chancellor Angela Merkel of Germany has also expressed a strong hope that Britain would not leave the bloc. So determined are some Germans that Britain remain that a group of German entrepreneurs, under the banner of “Bratwurst against Brexit,” came to London this month to give away sausages. They apparently were unaware that handing out food as part of an electoral campaign is against the law.

In France, a longtime ideological rival to Britain in the European Union, the country’s political establishment has backed Britain’s membership in the bloc. In an editorial last year written in English, the newspaper Le Monde made a stark warning to Britons: “Beware, Brexit could be your Waterloo!”

President François Hollande of France reiterated his preference for British membership on Thursday, saying that a withdrawal would have “very serious consequences” for all parties, and he pointedly noted that such a vote would be irreversible.

But that has not stopped euroskeptic politicians like the far-right leader Marine Le Pen from seeking to gain political capital from the vote.

On Tuesday, Ms. Le Pen proclaimed her support for a British exit and called for a “Frexit” from the European Union, which she characterized as “decaying.” Some of her fellow citizens tried to show solidarity with Britons by handing out croissants in London the next day, but they were rebuffed under the same law that put a damper on the bratwurst group.

In Poland, the hulking Palace of Culture and Science in Warsaw was illuminated this week with the Union Jack colors to show support for Britain membership. Although Poland’s relationship with Brussels has been acrimonious of late, its president, Andrzej Duda, recently warned that the European Union could collapse if Britain left.

In Brussels, the capital of the European Union, the prospect of a British exit is causing a headache, not just for bureaucrats but at Mini-Europe, a Brussels theme park that features mini replicas of the Eiffel Tower, the Acropolis and Britain’s Houses of Parliament. It recently spent 120,000 euros, about $137,000, to renovate its version of Big Ben.

Philippe Steenbergen, director of marketing, lamented that if Britain voted to exit the European Union, the theme park would eventually have to remove its popular Britain exhibit.

He noted proudly that the park had used gold leaf in its Big Ben revamp. “The park showcases countries in the E.U. We don’t have Switzerland, or Iceland, or Turkey,” he said by phone. “It would be a real shame if Britain had to go.”