In its 75-year history, Le Journal du Dimanche, France’s leading Sunday newspaper, has almost never missed publication. But its operations ground to a halt this week after an editor with a far-right track record was abruptly appointed just ahead of a takeover of the paper by the French billionaire Vincent Bolloré, prompting a mass walkout by journalists and igniting a firestorm in French media and political circles.

Mr. Bolloré, an industrialist often described as France’s Rupert Murdoch, has been steadily building a conservative media empire, anchored by a Fox-style news network, CNews. The appointment of the editor, Geoffroy Lejeune, who was formerly at a far-right magazine that was fined for racist insults, raised concerns that one of France’s most prominent newspapers could be transformed into a right-wing platform.

“For the first time in France since the Liberation, a large national media will be run by a far-right personality,” said an open letter published this week in Le Monde, France’s biggest newspaper, signed by 400 academics, economists, cultural figures and left-leaning politicians backing the JDD, as the paper is known. “This is a dangerous precedent which concerns us all,” the letter said.

Journalists at the JDD, known for its interviews with government leaders and largely centrist policy analysis, voted on Thursday to prolong their walkout to protest the hiring of Mr. Lejeune, 34, who was fired last year from the magazine Valeurs Actuelles amid a dispute with the owner over editorial direction. The paper did not publish Sunday, only the second time since its founding in 1948, and on Thursday evening the website was still leading with news from last week.

Over 1,000 people gathered at a theater in Paris this week at a rally organized by Reporters Without Borders, which condemned what it said was an effort by Mr. Bolloré to assert shareholder control over a newsroom.

France’s culture minister, Rima Abdul Malak, weighed in on Twitter. “Legally, the JDD can become what it wants, as long as it respects the law,” she wrote. “But as far as our Republic’s values are concerned, how can one not be alarmed?”

The episode has turned a fresh spotlight on Mr. Bolloré, a politically connected industrialist who hails from traditionalist Catholic circles in Brittany. His business empire includes the global advertising agency Havas, and he has a controlling stake in the media conglomerate Vivendi. He forged his fortune in logistics and was known as the King of Africa for the vast business dealings that earned him riches in former French colonies.

After a corruption inquiry into accusations that he helped the presidents of two African nations gain power in exchange for lucrative business contracts, Mr. Bolloré shifted focus in recent years to his news media properties, which in France tend to be a main avenue for the very rich to influence political elections. Over four-fifths of privately owned newspapers and TV and radio stations in France are owned by French or foreign billionaires or financiers. French state-backed television and radio stations hold dominant positions in the media landscape.

Pending approval of European Commission antitrust authorities, Mr. Bolloré is set this summer to secure his majority stake in Lagardère, a conglomerate that owns the JDD and Paris Match magazine. It would make him head of one of the largest broadcast and print empires in France.

Arnaud Lagardère, the conglomerate’s chief executive, who essentially now reports to Mr. Bolloré, sought this week to assuage concerns over the hiring of Mr. Lejeune, who has not issued any public statements other than a brief Twitter message saying he was honored to take the helm. Mr. Lagardère said the hiring decision, which he insisted was his alone, was purely a business choice and not intended to change the editorial line.

“This fantasy that the extreme right is working its way into the paper is not real,” he told the newspaper Le Figaro. But he added: “The JDD must also know how to adapt to changes in the world.”

Mr. Lejeune wrote on Twitter last week that his appointment was a “huge honor” and that he would put “all my energy into the success of this challenge.” He did not respond to a request for comment.

Under Mr. Bolloré, who typically avoids interviews and did not respond to a request for comment, several mainstream news outlets have been transformed into right-leaning platforms that analysts say align with his political convictions and a personal concern that Christian culture is eroding in France. He recently bought a flailing Christian newspaper with fewer than 10,000 subscribers, La France Catholique, with the aim of growing it.

The biggest shift has taken place at CNews, once a 24-hour news network, where many journalists were ousted or resigned in protest when Mr. Bolloré became owner in 2015. Their replacements shifted the focus to opinion segments and debates over hot-button issues, like crime, immigration and Islam’s role in France.

The makeover propelled CNews to the top-rated TV spot in France, a country that has seen a steady rise in influence among right and far-right politicians, especially in last year’s presidential election.

CNews gave a bullhorn to figures like Éric Zemmour, a best-selling author known for far-right nationalism, including the conspiracy theory of a “great replacement” of white people in France by immigrants from Africa. Inspired by Donald J. Trump, Mr. Zemmour became a TV star on CNews and ran against President Emmanuel Macron and Marine LePen in last year’s presidential election, in an ultimately unsuccessful bid.

Similar rightward swings at Mr. Bolloré’s other media holdings, including a Canal Plus news channel and Europe 1, a top-rated radio station, led to departures of reporters and editors.

So when journalists at the JDD learned of Mr. Lejeune’s appointment — not through an official announcement but via a news report — a revolt broke out in the newsroom.

“Journalists are very worried about media independence,” said Julia Cagé, an economist specializing in the media at Sciences Po, a research university in Paris.

“If you look at what happened for the past 10 years, Bolloré has destroyed the media he bought, and used them to push a radical right-wing line, anti-rights for minorities and a Catholic perspective,” she said. “In that sense, he’s become worse than Rupert Murdoch.”

But in a country where right-wing candidates received over 30 percent of the vote in last year’s presidential election, Mr. Bolloré’s platforms have filled what his supporters say was a political void in a French media landscape dominated by politically correct, left-leaning journalists.

“The media space in France is not neutral,” said Dominique Reynié, a professor at Sciences Po and the founder of Fondapol, a right-leaning think tank. “If you bring up issues like immigration or Islamism, which really are problems in France, you are badly received by journalists who consider you to be right or extreme right.”

Mr. Lejeune’s appointment was a reflection of how France’s media landscape is shifting in the direction of “what is happening in France electorally, which is an increasing shift to the right,” Mr. Reynié added. “There is a readership market on that side, which is not reading the left-leaning press.”

That is a bet that Mr. Bolloré seems eager to take.

“We have other media that is owned by industrialists who don’t interfere with the editorial line, which is not the case with Bolloré,” said Christian Delporte, a media historian at the University of Versailles.

“If he buys media, it’s because he has in mind the desire to influence the political future of the country,” he said. “He is accompanying the rise of the far right to power.”