A Frenchman turning on his television set one night last week could have gotten the alarming impression that the Russians were coming.

There were Soviet tanks rolling across West Germany, U.S. troops falling back in disarray, and a grave-faced President Francois Mitterrand announcing that he was considering using nuclear weapons to stop an imminent Soviet invasion. The program implied that he decided against this: viewers later saw Soviet security agents chasing French resisters through the Paris metro.

The show, hosted by Yves Montand and using old newsfilm to present hypothetical war scenarios, provoked a torrent of protests from the Kremlin. It has also touched off a domestic debate about the credibility of the French nuclear deterrent, the so-called force de frappe.

The controversy is significant because it touches on a subject that long has been regarded as off-limits by French politicians. The possession of an independent nuclear strike force capable of deterring any potential aggressor was enshrined as the nation's basic defense doctrine by Charles de Gaulle in the early 1960s and has since been accepted by all political parties including the Communists.

In contrast to most other Western European countries, in France the major challenge to conventional defense wisdom has come not from the left but from the right. Far from calling for unilateral disarmament, the critics are arguing that successive French governments have failed to keep pace with new kinds of Soviet military threat, particularly in the field of conventional and chemical weapons.

"The force de frappe is completely credible only against a massive Soviet nuclear attack," said Pierre Lellouche, a participant in last week's television program and author of a new book entitled "The Future of War." "We have to look for other remedies as well if we don't want to be faced with a situation of 'all or nothing.' "

The French government, Lellouche added, should gear up to fighting a future defensive war on the Elbe River -- which separates Eastern and Western Europe -- rather than the Rhine, which marks France's frontier with West Germany. This would mean openly moving away from the Gaullist notion of relying on the force de frappe to defend the "sanctuary" of French national territory.

A key argument in the television show was that France would have to bear a much heavier defense burden if American troops left West Germany. Montand also told viewers that President Reagan's "Star Wars" initiative might eventually leave Western Europe at the mercy of the two superpowers.

"Europe would be totally disarmed, naked," he said. "If both the two superpowers managed to construct such a defensive system, then our deterrent wouldn't have much sense since . . . all, or almost all, our missiles would be intercepted and diverted."

The program was criticized as alarmist and simplistic by Defense Minister Charles Hernu, who expressed confidence in the continued effectiveness of the force de frappe.

In another move in the battle for public opinion, the French Defense Ministry last week took journalists to the nuclear submarine base at Ile Longue near Brest on the Brittany peninsula in northwest France. The aim of the visit evidently was to demonstrate that the French nuclear strike force is keeping up with technological advances in the Soviet Union and the United States.

Standing on the deck of Le Foudroyant (The Thunderbolt), a naval officer explained that each of the 16 nuclear warheads lodged in silos beneath his feet contained 50 times the destructive power of the bomb dropped on Hiroshima. By the end of the decade, he said, the now-outdated M20 missiles aboard the submarine will be replaced by modern M4 missiles with six independently targetable nuclear warheads each and a range of over 2,500 miles.

By 1990, France's nuclear strike force will have expanded from 118 warheads to 436. The sixth and latest French missile-carrying nuclear submarine, L'Inflexible, already is equipped with M4 multiple warhead missiles.

"The credibility of our force rests on new technology," said Vice Adm. Alain Coatanea, who recently became commander of the strategic submarine force, the most important element in the French nuclear triad of air-, land-, and sea-based missiles.

For the critics, all the talk about the modernization of the force de frappe misses the essential point: that France lacks the flexibility to respond to a nonnuclear threat from the Soviet Union.

"We don't have the right to allow the independence of our country to rest on the hope that, when the crunch comes, the Soviets will believe that we are ready to commit suicide," television viewers were told by Gen. Etienne Coppel, who resigned from France's Air Force last year to protest the government's defense policies.

The suggestion that French defense strategy rests on the all-or-nothing approach is rejected by Hernu and other military leaders. They cite the recent creation of a rapid deployment force of 47,000 men, which has as one of its aims to reinforce NATO forces in forward positions in West Germany in case of Soviet attack. France is also modernizing its force of tactical nuclear weapons.

Under French defense doctrine, nuclear weapons can be used in the event of a threat to France's "vital interests." French strategists have refused to define these "vital interests" precisely, on the grounds that keeping the aggressor guessing is a key element in the effectiveness of deterrence.

Responding to the television show, Hernu said: "Who can say that one would not reply to a chemical war with a nuclear war? That is the marvelous uncertainty of nuclear deterrence."