France's leading press magnate, "Citizen Hersant" for those sections of the mass media he does not yet control, is back in the news again with a dramatic gesture of political defiance.

Legally prevented by the Socialist government from expanding his newspaper empire, Robert Hersant has provoked a mixture of outrage and admiration here by doing precisely that. His acquisition earlier this month of yet another newspaper group has provided a new twist to one of the longest running feuds in French politics.

Vehemently right-wing, Hersant, 66, is a man the French left loves to hate. When the idea of press anti-trust legislation was floated at a Socialist Party congress in 1983, ecstatic cheering erupted at the thought of clipping the ambitions of France's Citizen Kane (the movie character based on the U.S. newspaper baron William Randolph Hearst). Here at last was an issue on which every card-carrying Socialist could agree.

The feelings are reciprocated. Since President Francois Mitterrand's left-wing administration came to power in 1981, Hersant's newspapers have kept up an unremitting barrage of criticism, accusing the government of presiding over France's political and economic ruin. The group's flagship, Le Figaro, has become a virtual mouthpiece for the right-wing opposition.

The paradox here is that, far from falling on hard times, Hersant and his newspapers have prospered under left-wing rule. The group's acquisition this month of eight newspapers in the southeastern region of Rhones-Alpes gives Hersant control of 26 percent of the French provincial press and 38 percent of the national press. He already owns 30 local radio stations, a news agency, and a chain of general-interest periodicals.

Under the press law passed last year by the Socialist-controlled National Assembly, no one person is allowed to own more than 15 percent of the total circulation of either national or regional newspapers.

With just two months to go before nationwide legislative elections that are widely expected to result in a defeat for the Socialists, Hersant evidently feels that he can flout the law. The right-wing opposition has promised to repeal the legislation if it gets back into power.

Noting that Hersant was behaving as if the left already were beaten, a front-page editorial in the Paris newspaper Le Monde described the new press law as "a mountain that has given birth to a mouse."

"The means at the government's disposal are so derisory that it is clear that the publisher of Le Figaro is going to win this particular battle," predicted Le Monde's editor, Andre Fontaine. "His political influence will be strengthened as a result."

The government promptly announced that it would bring legal charges against Hersant, and a governmental antitrust commission, which has only advisory power, declared today that his latest purchase was illegal. But the government's hopes of satisfaction appeared slim. As a member of the European Parliament, the newspaper magnate enjoys immunity from criminal prosecution. The government can ask for his immunity to be lifted, but there is practically no chance of such a step being approved by mid-March.

The political controversy surrounding Hersant goes back to World War II, when he actively supported marshal Philippe Petain's Vichy regime that collaborated with Nazi Germany. In August 1940, he helped found a "national propaganda center" -- a movement described by a Vichy newspaper of the time as "anti-Jewish" and "anti-Masonic."

After the war, Hersant was sentenced to 10 years' "national indignity" as a collaborator. This involved the loss of civil rights, such as the right to hold office. He was amnestied in 1952.

Hersant's career as a press magnate began in 1950, when he started up a car magazine, L'Auto-Journal, that attracted readers by publishing blueprints of new automobiles. His strategy for building up his newspaper empire has been simple. He buys up failing newspapers and makes them profitable by cutting costs, reorganizing the management and laying off staff.

As in the cases of other prominent newspaper proprietors, including Australia's Rupert Murdoch and the late Axel Springer of West Germany, opinions are sharply divided about Hersant. For his critics, he is a threat to the pluralism of the French press. For his admirers, he is the savior of papers that otherwise would have perished.

Right-wing spokesmen have defended Hersant's empire-building by pointing out that the biggest press monopoly in France is run by the government, which controls all three television channels, several radio stations, and one of Western Europe's most powerful advertising agencies, Havas. They also have accused the government of trying to divert attention from its decision to award the contract for a fourth television channel to a swashbuckling Italian entrepreneur, Silvio Berlusconi.

In a front-page editorial in Le Figaro, Hersant asked what the country was coming to if the government was ready to do business with foreigners while discriminating against good French citizens such as himself. He charged that the real issue was the freedom of French citizens "to express themselves freely" and added: "Sometimes in order not to be late for a war, it is necessary to be in advance of a law."

What makes Hersant different from other press magnates is the way he openly mixes politics and journalism. His political ambitions are likely to receive a further boost if, as expected, he is elected to the National Assembly along with a dozen close associates next March on a right-wing ticket.

Hersant, who has staked everything on the return of the right, already is being mentioned as the leading candidate for the purchase of a state-run television channel slated for denationalization by the opposition.

Socialist Party Secretary Lionel Jospin warned: "For the Socialist Party, Robert Hersant is an opponent. For the right, he risks becoming a boss."