LONDON — From Brussels to Berlin to Washington, leaders of the Western democratic world awoke Friday morning to a blunt, once-unthinkable rebuke delivered by the flinty citizens of a small island nation in the North Atlantic. Populist anger against the established political order had finally boiled over.

The British had rebelled.

Their stunning vote to leave the European Union presents a political, economic and existential crisis for a bloc already reeling from entrenched problems. But the thumb-in-your-eye message is hardly limited to Britain. The same yawning gap between the elite and mass opinion is fueling a populist backlash in Austria, France, Germany and elsewhere on the Continent — as well as in the United States.

The symbolism of trans-Atlantic insurrection was rich on Friday: Donald J. Trump, the presumptive Republican presidential nominee and embodiment of American fury, happened to be visiting Britain.

“Basically, they took back their country,” Mr. Trump said Friday morning from Scotland, where he was promoting his golf courses. “That’s a good thing.”