George F. Kennan, the former U.S. ambassador to the Soviet Union and Pulitzer Prize-winning historian, yesterday called for basic shifts in U.S. national thinking and policy to halt what he called "a march toward war" between the nuclear superpowers.

Speaking to a luncheon meeting of the American Committee on East-West Accord, of which he is co-chairman, Kennan, 79, said that as a result of several years of deterioration Soviet-American relations are in "a dreadful and dangerous condition."

According to the retired diplomat, whose 55 years of involvement with the Soviet Union exceeds that of almost anyone still in public life, the combination of deepened political hostility and ever more powerful nuclear weaponry "present a shadow greater than any that has ever darkened the future of western civilization."

Kennan stopped just short of saying that war between the two nations is inevitable, noting the "antagonism, suspicion and cynicism" that he said exist between them and the increasing militarization of policy.

"Historically speaking, these phenomena . . . , when they occur in the relations between highly armed great powers, are the familiar characteristics, the unfailing characteristics, of a march toward war, that and nothing else," Kennan said.

In May, 1981, in a speech here also notable for its pessimism, Kennan proposed that the two nations reduce their atomic arsenals by 50 percent to break the momentum in the arms race.

The diplomat-historian said after his address yesterday that he was "delighted" by President Reagan's proposal in November, 1981, for deep cuts in U.S. and Soviet strategic nuclear arsenals. In the speech, he described as "useful" deep cuts, the nuclear weapons freeze resolution, a comprehensive test ban treaty and the "build-down" nuclear arms proposal under discussion.

To make progress on any of these plans, he added, "we would have to treat the problem as a whole in our talks with the Russians--not in a series of fragmentized technical negotiations--and to treat it at the senior political level."

Kennan's central point was that the prevailing U.S. official view of the Soviet Union is "grotesquely overdrawn" and "highly misleading and pernicious as a foundation for national policy." At another point he spoke of the deep-dyed view of the Soviets as "inexcusably childish, unworthy of people charged with the responsibility for conducting the affairs of a great power in an endangered world."

He went on to acknowledge negative characteristics of the Soviet regime, including "a high general sense of insecurity, a positively neurotic passion for secrecy, a marked sensitivity to conditions in border regions and a tendency to overdo in the cultivation of armed force."

Kennan said most of these characteristics are "actually less acute than they were many years ago." Moreover, he said they are balanced by other factors, especially indications that the Soviet leadership "will, given a chance, go quite far with us to avoid" war.

He called for such U.S. steps as improving communications and ressurrecting detente-era agreements, to make the Washington-Moscow relationship constructive.