BUDAPEST — Emboldened by encouraging signals from the Trump administration, populist leaders across Central and Eastern Europe are mounting simultaneous crackdowns on nongovernmental organizations, once protected by Washington, that promote open government, aid refugees and often serve as checks on authoritarian governments.

In Hungary, where the movement has reached a fever pitch, supporters of Prime Minister Viktor Orban are vilifying “foreign-funded” N.G.O.s — especially those succored by George Soros, the liberal American billionaire — and accusing the groups of wanting to flood Europe with Muslim refugees and transform “Christian” nations into multicultural stews of left-wing globalism. Earlier this week, Zoltan Kovacs, Mr. Orban’s chief international spokesman, described the organizations as “foreign agents financed by foreign money.”

Macedonia’s former autocratic prime minister, Nikola Gruevski, has called for a “de-Sorosization” of society, labeling opponents “Soros-oids” and inspiring a “Stop Operation Soros” movement in January. Poland’s governing party leader, Jaroslaw Kaczynski, says Soros-funded groups want “societies without identity,” and backs fresh efforts to regulate them. In Romania, where hundreds of thousands of anticorruption protesters took to the streets in recent weeks, the leader of the governing party charged that Mr. Soros “financed evil” and has vowed to defeat him. Similar efforts have begun or accelerated in Serbia, Slovakia and Bulgaria since Mr. Trump’s victory.

“These organizations must be pushed back with all available tools,” Szilard Nemeth, vice chairman of Hungary’s governing Fidesz party, told journalists. “I think they must be swept out, and now I believe the international conditions are right for this with the election of a new president.”