Lech Walesa, the leader of Poland’s Solidarity movement, which helped end Moscow’s grip on Eastern Europe at the end of the Cold War, joined with former Polish political prisoners on Monday to send an impassioned letter to President Trump voicing “horror and disgust” at his scolding of President Volodymyr Zelensky of Ukraine last week, saying it reminded them of their encounters with bullying Communist-era officials.

They wrote in Polish that they were “terrified by the fact that the atmosphere in the Oval Office during this conversation reminded us of the one we remember well from interrogations by the Security Service and from courtrooms in Communist courts.”

“Prosecutors and judges, commissioned by the all-powerful communist political police, also explained to us that they held all the cards and we had none,” the letter said, a reference to President Trump’s Oval Office rebuke to Mr. Zelensky that “you don’t have the cards.”

Communist functionaries, the letter continued, “demanded that we stop our activities, arguing that thousands of innocent people were suffering because of us.” When President Zelensky insisted in the Oval Office on Friday that security guarantees were needed to make any peace deal with Russia last, Mr. Trump slapped him down, saying, “You’re gambling with the lives of millions of people.”

The letter — signed by Mr. Walesa, the 1980s leader of the Solidarity trade union, and more than 30 prominent former Polish political detainees — was posted on Mr. Walesa’s Facebook page, along with a sometimes imprecise English translation and an old photograph of him meeting with a grinning, tuxedo-clad Mr. Trump.

It expressed angry disbelief that Mr. Trump and Vice President JD Vance had berated Mr. Zelensky for not thanking them enough for helping Ukraine.

“Gratitude is due to the heroic Ukrainian soldiers who shed their blood in defense of the values of the free world,” Mr. Walesa, who served as Poland’s first elected president after the collapse of Communism, and other signatories said, adding, “We do not understand how the leader of a country that is a symbol of the free world cannot see this.”

While many European leaders were dismayed and deeply alarmed by Mr. Zelensky’s treatment in the Oval Office, they have avoided criticizing Mr. Trump in public, fearful of stirring his wrath and deepening his anger at Ukraine. Mr. Walesa’s letter brought Europe’s feelings into the open, particularly its alarm that the United States under Mr. Trump is veering away from standing up to dictatorial bullies to side with them.

The letter recalled the vital role that President Ronald Reagan had played in supporting Moscow’s opponents in the 1980s and bringing about the collapse of the Soviet Union. “President Reagan was aware that in Soviet Russia and the countries it conquered, millions of enslaved people suffered, including thousands of political prisoners who paid for their sacrifice in defense of democratic values with freedom,” it said.

Pleading for the United States not to turn its back on decades of support for opponents of tyranny, the letter warned, “The history of the 20th century shows that every time the United States wanted to maintain distance from democratic values ​​and its European allies, it ended up threatening itself.”

Anatol Magdziarz contributed reporting.