The French state monopoly over television ended tonight with the controversial launching of the country's first private television station just four weeks before crucial legislative elections.

Millions of television viewers tuned in to the new channel, known simply as "La Cinq" (Channel Five), which went on the air with a glitzy, American-style variety show produced entirely in Italy.

Run by a swashbuckling Italian entrepreneur in collaboration with a left-wing French industrialist, the new channel forms part of a future audiovisual revolution in Western Europe as the old state-dominated monopolies break apart. Attracted by the world's largest potential television audience, competing multinational companies have been forming with the ultimate aim of broadcasting by satellite across the continent.

"The real challenge is to create an authentically European television service which could compete with that of the United States, which otherwise threatens to submerge us," Silvio Berlusconi, the Italian director of the new station, said in a recent interview.

A dynamic businessman, Berlusconi built up the largest private television network in Italy by adopting the American formula of quiz shows, movies and variety programs interspersed with frequent advertisements. Known as "Sua Emittenza" (His Emittence) in Italy, he has already announced plans to expand his television empire to Spain as well as France.

The hurried launching of La Cinq has been widely interpreted in France as an attempt by President Franc,ois Mitterrand's Socialist government to preempt right-wing plans for the immediate denationalization of two state-run television channels in the event of an opposition victory next month. Up until four months ago, most Socialist leaders were outspoken in their criticism of the "Coca-Cola television" personified by Berlusconi.

The neo-Gaullist mayor of Paris, Jacques Chirac, sought to block today's launching by refusing the new television station permission to set up transmitters at the top of the 984-foot Eiffel Tower, which in theory he controls. The government fought back, however, by sending in uniformed police to enforce new legislation granting state-approved broadcasting organizations the right to place transmitters anywhere they wished.

Another right-wing leader, former prime minister Raymond Barre, this week accused Mitterrand of arbitrarily distributing television and radio licenses to his friends as if he were some kind of prince.

The decision to grant a television license to Berlusconi was also strongly criticized by leading writers and filmmakers, who warned that it could result in a sharp decline in standards. Culture Minister Jack Lang expressed fears about the threat to the France's thriving cinema industry if the new channel goes ahead with its plans to broadcast large numbers of movies.

Tonight's gala launch, which cost nearly $1 million to produce, featured an array of celebrities, including ballet dancers such as Rudolf Nureyev, film stars such as Sean Connery and Ornella Muti, and France's best-known football player, Michel Platini. There was also an advertising spot for an Italian brand of pasta featuring the movie director Federico Fellini.

Socialist Party spokesmen have sought to depict the launching of La Cinq as part of the liberalization of government controls over broadcasting that began with the left-wing election victory of May 1981. Hundreds of private radio stations have already sprung up around the country as a result of a 1982 law ending the state monopoly over the airwaves.

Before 1981, both radio and television were regarded as an essential public relations instrument for whatever government happened to be in power at the time. The traditional French attitude toward the audiovisual media was summed up by a former culture minister, Andre Malraux, who is reported to have asked President Kennedy in some amazement, "How can you govern a country if you don't control television?"

Recently, however, political parties of both left and right have promised to end the state monopoly over television. The right-wing election program includes a pledge to denationalize two out of the three existing state-run channels and one semipublic pay TV channel as soon as possible after the elections.

Political analysts said the launch of La Cinq could upset these plans in view of the limited advertising market. Yesterday Chirac himself suggested in a speech that a new government might have to delay the politically symbolic denationalizations to allow more time for detailed financial studies.

The government has already approved proposals for two more stations following tonight's launching of La Cinq. La Six, which is due to begin broadcasting this weekend, will be a private music station featuring rock videos, movies and variety shows.

A state-run seventh channel, featuring more serious documentaries and cultural programs, is expected to start broadcasting in the fall.

In addition to the new television stations, the government has also announced plans for a satellite that will be launched later this year. Berlusconi has been promised one of four channels on the satellite, which will have the potential of beaming programs to around 200 million viewers across Western Europe starting next year.