Historian and former diplomat George F. Kennan, declaring that the United States and the Soviet Union are on a "collision course" toward eventual nuclear war, proposed yesterday that the two superpowers immediately reduce their atomic arsenals by 50 percent to break the momentum.

Kennan, whose description of Soviet conduct in 1947 helped fashion the U.S. postwar policy of "containment" of the Soviets, made the proposals while accepting the $50,000 Albert Einstein Peace Prize at a luncheon here.

Only "a bold and sweeping departure" from present military and political trends can go to "the heart of the problem" of the nuclear weapons buildup that imperils both countries, Kennan said.

The former U.S. ambassador to Moscow and winner of the Pulitzer Prize for literature told a press conference after his speech that he realizes his proposal appears drastic and would take time for serious study, to say nothing of acceptance, by the administration.

Kennan said he had no indication from either the U.S. or Soviet governments that such a plan would be accepted, but he expressed the belief that in the end governments and people will not allow the world to "drift along" toward disaster.

Before an audience that ranged from Soviet Ambassador Anatoliy F. Dobrynin to National Security Council aide and Soviet critic Richard Pipes, and included many former U.S. arms control officials, Kennan expressed a lack of confidence in the strategic arms negotiations pursued by Washington and Moscow throughout the 1970s.

While expressing regret that the SALT II agreement was not ratified by the Senate, Kennan said, "I have no illusion that such negotiations could ever be adequate to get us out of this hole. They are not a way of escape from the nuclear weapons race; they are an integral part of it."

Instead, the 77-year-old scholar called on President Reagan, after consultation with Congress, to propose to the Soviet government "an immediate across-the-board reduction by 50 percent of the nuclear arsenals now being maintained by the two superpowers -- a reduction affecting in equal measure all forms of the weapons, strategic, medium-range and tactical, as well as all their means of delivery -- all this to be implemented at once and without further wrangling among the experts and to be subject to such national means of verficiation as now lie at the disposal of the tow governments."

Kennan quoted with approval Reagan's statement that he would "negotiate as long as necessary to reduce the numbers of nuclear weapons to a point where neither side threatens the survival of the other." But a shortcut is necessary to avert "the same old fateful track that has brought us where we are today," Kennan added.

"We have gone on piling weapon upon weapon, missile upon missile . . . helplessly, almost involuntarily, like the victims of some sort of hypnosis, like men in a dream, like lemmings headed for the sea" until the total numbers have reached "such grotesque dimensions as to defy rational understanding," Kennan said.

He added that the two nations have amassed in their nuclear arsenals more than one million times the destructive power of the atomic bomb that leveled Hiroshima in 1945.

Kennan said, on the basis of information from American scientists, that "something well less than 20 percent" of the existing stocks would be sufficient for deterrence, if the relative proportions of the weaponry in Soviet and American hands were preserved.

The Albert Einstein Peace Prize Foundation, a private group founded two years ago on the 100th anniversary of the famous mathematician's birth, announced it is beginning a drive for 10 million signatures on a manifesto calling for reductions in U.S. and Soviet nuclear stockpiles.