France will reject efforts by the Reagan administration to enlist Western Europe in a campaign of economic warfare against the Soviet Union, President Francois Mitterrand has declared.

In an interview at the Elysee Palace, Mitterrand stressed that France was prepared to cooperate with the United States in "defensive measures" against the Soviets "to contain their ambitions, to resist their advances, to block their penetration." But he firmly came out against a strategy based on the notion that trade and financial restrictions will seriously undermine Soviet determination and strength.

President Reagan, Defense Secretary Caspar W. Weinberger and national security adviser William P. Clark are among the administration officials who have laid out such a strategy in public statements asserting that the Soviet economy is crumbling and vulnerable to such pressures. This view was a cornerstone of American proposals at the economic summit in Versailles and the Bonn NATO summit last week.

"We are not going to wage any kind of war on the Russians," the French Socialist leader said. "You have to be very serious about such a course. It could lead to a real war. If economic embargo is a first act of war, it risks being caught up by a second. No, it is not the right move. Obviously, at the same time we will do nothing that needlessly builds up Soviet military potential."

In his first year in power, Mitterrand has given surprisingly strong public support to the Reagan administration's strategic rearmament program, and that support has helped defuse leftist opposition to the scheduled deployment of a new generation of U.S. missiles in Europe. At several points in the 45-minute discussion Friday afternoon, he voiced the strongest commitment to NATO that any French leader has given since Charles de Gaulle withdrew from the alliance's integrated military command.

But Mitterrand's comments suggested that the two summit meetings had left a wide and possibly growing gap between American and European assessments of how to deal with the Soviet Union. The overall tone, rather than anything specific he said, raised the possibility that French-American cooperaton on East-West issues now may have reached its limits after a surprisingly harmonious year.

Despite a demanding schedule in recent weeks that included hosting the Versailles summit, a trip to Bonn for a dinner preceding the NATO meeting, and a two-hour press conference, the 65-year-old Socialist leader appeared in a relaxed, at times impish mood, vigorously turning back questions he wanted to avoid with deft irony.

There were also some signs of close-to-the surface irritation over the failure of the meeting of the seven major industrialized non-communist countries at Versailles to go very far in lowering what Mitterrand called "their internal competition, which is going on in unacceptable ways." He dismissed unnamed American officials who, he said, had agreed inside the summit to study intervention in exchange markets and then held press conferences saying that the agreement had no importance.

The discussion between Mitterrand and two American journalists took place immediately after the president had met with Saudi Foreign Minister Saud al-Faisal on the Lebanon crisis, and just before a session with his prime minister, Pierre Mauroy, to discuss emergency measures to shore up the French economy, plagued by 14 percent inflation, a falling franc and high unemployment. On these and other subjects, Mitterrand made these points:

He continues to be far more friendly toward Israel than any of his predecessors, despite what he sees as the "error" of the invasion of Lebanon. He said even if Israel succeeded in eliminating the Palestine Liberation Organization as a military and political force, "it would do nothing to change the right of a people to have a country. That would be a brutal kind of reasoning." Mitterrand again last week repeated that he favors a Palestinian entity centered on the occupied West Bank and Gaza territories.

He suggested that the United States and the Soviet Union bear some of the responsibility for the latest Middle East tensions by "sparring with each other like boxers" through their regional allies, Israel and Syria. "I have the impression that the two superpowers have been letting their allies go just as far as they can, up to the very point where it would be going too far . . . . There are limits. And with Israel in this moment, the limits seem large . . . . It is not a question of powerlessness by the superpowers."

* French-West German security discussions are producing closer understanding on defense strategies. Mitterrand declined to answer directly when asked if that understanding would include a French commitment to use its nuclear arsenal to protect West Germany's existence in a dire emergency, but he did underline that "it would be stupid for anyone to think that France would be indifferent to Germany's fate."

* His government will stress continuity in its relations with former French colonies in Africa, which will continue to be "a privileged area" for French aid and interest under the Socialists. "We are not going to waste what has been gained there."

* He acknowledged that even after the vigorous pump-priming and nationalizations carried out by the Socialists in their first year of power, the French economy remains "a tired economy" that must find ways to compete more strongly internationally and to regain its own domestic markets. But he made it clear that he would leave discussion and implementation of austerity measures up to Mauroy and his Cabinet.

Mitterrand declined to talk about his discussions about the Soviet Union with Reagan and the leaders of Britain, Canada, Italy, Japan and West Germany at Versailles. But other officials involved in the summit portrayed Reagan's remarks as suggesting that many Soviet citizens were close to starvation because of economic mismanagement, and that the Soviet system could be brought down if Europe would cut back sharply on trade and credits.

The United States came to Versailles pushing for reductions in government-subsidized credits to the Soviet Union given primarily by France and Italy, but settled for a summit declaration that spoke only of the need to limit all kinds of credit granted to the Soviet Union. Mitterrand said Friday that the limitations had already been accomplished by France by raising its interest rates, and that France was not bound by the summit declaration to reduce the current amount of credit extended to the Russians.

"We have accepted limitations on exports of high technology products" as determined by NATO's Coordinating Committee for Multilateral Export Controls, which is known as Cocom. "We do not want to be part of an economic Cocom. Anyway, it is ridiculous. How could French and Italian public credits possibly contribute to a Soviet military buildup?" Economic restrictions "would have to be applied to all exports, whether they are paid on delivery or on credit. Then you see where it would go, non?" The Soviet Union pays for grain deliveries from the United States in cash.

Asked if the appearance that Leonid Brezhnev's age and health might soon lead to a change in the Soviet leadership was affecting Soviet dealings with the West, Mitterrand said:

"I think that the Soviet position largely depends on their desire to bring the Geneva talks on European theater nuclear missiles one step further and that other Soviet imperatives are put behind this leading concern about the Geneva talks. That is perhaps why possible Soviet actions are kept quiet. . . . The negotiations should be broader" than trading off U.S. Pershing II missiles due to be deployed in Europe in late 1983 against the Soviet SS20, "but if the Soviets give up a part of their armament, and if they accept a control, then the deployment won't take place. But the Russians must understand that if they refuse to start serious negotiations they will get the Pershings."