France's Socialist government stepped up its criticism of President Reagan's Strategic Defense Initiative today, predicting that it would lead to a new round in the nuclear arms race and weaken the security of Western Europe.

The sweeping French criticism of SDI was contained in a full-page interview by the Paris newspaper Le Monde of Defense Minister Paul Quiles on his return from talks in Washington with U.S. officials. It marked the most comprehensive critique of American plans for a space-age missile defense system yet delivered by a West European leader.

"SDI could upset the strategic concepts on which peace has rested since the last war," said Quiles, adding that it also could lead to a further polarization of public opinion in Western Europe between "those who take refuge in neutralism and pacifism" and "those who place their destiny in the hands of the superpowers."

Insisting that finding ways to counter a strategic defense system would prove much cheaper than building one in the first place, Quiles said that France would take the necessary measures to preserve the credibility of its own nuclear strike force.

The latest French criticism of SDI, or "Star Wars," as the project is popularly known, goes far beyond the reservations expressed by other West European countries, notably Britain and West Germany. The French position on SDI is now closer in some respects to that of the Soviet Union than that of the United States, according to diplomatic analysts here.

Britain has already signed a contract with the United States on participating in SDI research, despite its concern about some aspects of the program. The West German government announced today that Economics Minister Martin Bangemann will lead negotiations with the Reagan administration on a similar contract.

Differences over SDI emerged in talks here today between President Francois Mitterrand and West German Chancellor Helmut Kohl, according to French officials. Earlier this year, the two leaders attempted to coordinate a common West European position on Star Wars, but this effort now seems to have failed.

In the Le Monde interview, Quiles said that he had been struck by the "enthusiasm" and "determination" with which senior U.S. officials promoted SDI while he was in Washington last week. His interlocutors included Defense Secretary Caspar W. Weinberger, Secretary of State George P. Shultz, Undersecretary of Defense Fred C. Ikle, and Lt. Gen. James A. Abrahamson, director of the SDI project.

Making clear that he was not persuaded by the U.S. arguments in favor of SDI, Quiles listed French objections including its huge cost and questions about its technical feasibility. He said his doubts had been "reinforced" by the skepticism of several leading U.S. scientists.

"The most optimistic predictions do not allow us to consider this a credible project, even for the next half century. Science can make progress, sometimes very rapid progress, but it cannot work miracles," Quiles said.

The defense minister said that despite the doubtful feasibility of SDI, even the attempt to construct such a system carried considerable risks for the East-West balance of power. He said that the most likely reply to SDI would be "a new increase in the level of offensive arsenals" in view of the fact that offensive systems were cheaper than defensive ones.

"A space defense risks becoming a new Maginot Line more costly than all such previous military projects," Quiles said, in a reference to a supposedly impregnable defense system, built by the French Army before World War II against Germany, that proved useless.

Quiles said that by creating "false hopes" of a nuclear-free world, SDI could undermine the consensus on nuclear deterrence that he said exists in France.

Public opinion polls have shown widespread support here for the maintenance of a nuclear deterrent, or force de frappe, which is considered a symbol of the country's independence. In contrast to other West European countries, pacifists enjoy little political influence in France.

Quiles said that even if the United States found a way of shooting down long-range missiles in space, Western Europe still would be vulnerable to Soviet nuclear-armed aircraft and cruise missiles, which fly close to the surface. He said SDI would do nothing to counter the risk of a conventional attack by the Red Army with numerically superior forces to those of the North Atlantic Treaty Organization.

"The first consequence of SDI would be the appearance of two unequal security zones: the superpow-ers on one side and Europe on the other," he said.

Last month, Quiles announced that France was developing a program to counter a possible Soviet space-based missile defense system. The project includes research on miniaturized nuclear warheads which would be invisible to defensive radar systems.

Quiles has taken a harder line against SDI than his predecessor, Charles Hernu, who was forced out of office in September in the wake of a political scandal triggered by the sinking of a Greenpeace environmental ship by the French secret services.

Political analysts here say they think Quiles' statements are closely coordinated with Mitterrand. Earlier this year, Mitterrand formally ruled out official French participation in SDI research. France has tried to persuade its West European partners to join a civilian research program known as Eureka that covers much of the same ground as SDI.

Quiles said that he believed the real aim of SDI was to create a political consensus in the United States, ranging from pacifist movements and the churches -- who were impressed by promises to banish nuclear weapons -- to the military that saw an opportunity to increase the defense effort.