Amid a turbulent change of power, Poland’s main state television news channel went abruptly off the air on Wednesday, as the former governing party sent legislators and other supporters to the public broadcasting headquarters to try to prevent new management from taking over.

Members of the ousted former government staged a sit-in inside a building in southern Warsaw that houses the studios and offices of state television, including TVP Info, a news channel and website that served as a propaganda bullhorn for the right-wing Law and Justice Party during its eight years in power.

The protesters, led by Jaroslaw Kaczynski, Law and Justice’s combative chairman, tried to prevent the new, centrist administration of Prime Minister Donald Tusk from asserting control, accusing it of staging a “coup d’état” by firing loyalists of Law and Justice, which lost a general election in October.

With Mr. Kaczynski and his supporters vowing to “defend democracy” and block a change of management ordered by Mr. Tusk’s culture minister, technicians who support the new government yanked TVP Info off the air and disabled its website, which had been featuring appeals for resistance against an “illegal attack on public television” by Mr. Tusk.

The ruckus highlighted the turbulence facing the new government as it struggles to assert its authority over a wide range of state bodies, including the courts, that are stacked with die-hard Law and Justice supporters.

The struggle over public television could be a foretaste of a long period of disruptive trench warfare between a new government trying to break Law and Justice’s grip on those institutions, and the party’s holdovers trying to thwart it. When Law and Justice first took power in 2016 it imposed its own management on state television within days and met no resistance.

The evening news program on TVP was canceled on Wednesday, replaced by a promotional video for an upcoming series about World War II, and a statement by Marek Czyz, a newly appointed news anchor, promising “clean water” instead of “propaganda soup” once news bulletins resumed. TV Republika, a small private channel loyal to Law and Justice, broadcast footage of TVP holdouts holed up inside the public broadcaster’s news studio as they tried to rally support — evidence that the new management had yet to gain full control of the station.

“You can count on our determination in this matter, ” he said.

Poland’s highest court, the Constitutional Tribunal, whose president is an old friend and supporter of Mr. Kaczynski, last week issued an order, requested by Law and Justice legislators, demanding that the new government not make changes to public broadcasting management.

The tribunal, like many lower courts, public media and a national prosecutor’s office set up under the old government, was heavily politicized during the tenure of Law and Justice, purged of people independent of the party. But in October, the party lost its majority in Parliament to a coalition of opposition parties aligned with Mr. Tusk, a former prime minister.

Ignoring the Constitutional Tribunal’s order against making changes, Parliament passed a resolution on Tuesday mandating a management overhaul at Poland’s public broadcasting system, a network of regional and national radio and television channels.

Accusing the network of being “exclusively party media outlets carrying out propaganda work,” the resolution said there was “an urgent need” to restore the independence of state-funded media.

Mr. Tusk’s culture minister, Bartlomiej Sienkiewicz, on Wednesday announced that he had fired the heads and supervisory boards of TVP, the television arm of public broadcasting, Polish Radio and the national news agency, PAP. TVP Info is a 24-hour news channel under the umbrella of TVP.

Mr. Kaczynski, Poland’s de facto leader until his party’s defeat at the polls, responded with the sit-in and a noisy protest outside the broadcasting headquarters by Law and Justice supporters that featured chants of “Free Media” and “Down with communism.”

“We will not hand Poland over to the commies,” shouted one protester, echoing Law and Justice’s unfounded claim that the new government wants to restore the repressive system that existed during the Cold War.

In the October election, Law and Justice captured more seats in Parliament than any other party, but far fewer than the combined opposition parties that formed a majority coalition.

Mr. Kaczynski has claimed that means his party won, has repeatedly pledged to resist a change of power, and has described Mr. Tusk, a former senior European Union official in Brussels, as a “German agent” who threatened Poland’s existence as a sovereign and independent state.

“We are dealing with a coup d’état. Society must speak!” said Jacek Sasin, a Law and Justice member of Parliament who took part in Mr. Kaczynski’s sit-in.

Mariusz Kaminiski, who served as interior minister and coordinator of Poland’s secret service until the October election, also joined the protest and compared the new government’s moves to take control to efforts by the country’s former communist regime to crush Solidarity by declaring martial law on Dec. 13, 1981.

Claims by Law and Justice that it is replaying Solidarity’s heroic resistance to communism four decades ago, however, have been undermined by some of its own former supporters, including Marcin Wolski, a prominent television personality and former media executive who, after the election, accused TVP of creating “worse propaganda” than the communists.

Several of Law and Justice’s most vicious on-air attack dogs have gone quietly, including Danuta Holecka, who anchored the main evening news until she resigned last week after years of pouring bile on Mr. Tusk and his allies. Others, however, have vowed to hang on to their jobs.

Those resisting the change received encouragement on Wednesday from the National Media Council, a supervisory body set up by Law and Justice when it first came to power and still controlled by its appointees. In a statement, the council denounced the management overhaul ordered by the new culture minister as “illegal” and warned that “people who participate in the unlawful attack on public media taking place before our eyes should expect to face severe consequences under the provisions of the Penal Code.”

Anatol Magdziarz contributed reporting.