Deviating from key strategic doctrines he advocated as secretary of state, Henry Kissinger said here this weekend that Western Europe should not rely on traditional U.S. nuclear guarantees to defend the continent.

"Perhaps even today, but surely in the 1980s, the United States will no longer be in a strategic position to reduce a Soviet counterblow against the United States to tolerable levels," he said at a meeting here Saturday of Western defense experts.

Emerging Soviet nuclear superiority has made it impossible for the United States to knock out Soviet missiles and largely reduced the United States to the sole nuclear option of destroying Soviet cities, then seeing U.S. cities destroyed in turn, Kissinger said.

These alternatives could weaken any U.S. leader's resolve if confronted with a Soviet threat to Europe, Kissinger said.

"Of course he could threaten a fullscale strategic response, but will he do it? Is it a realistic course?" he asked.

Kissinger urged massive new defense spending on weaponry capable of hitting military targets and fighting nuclear wars, including tactical nuclear exchanges in Europe that would not escalate to massive retaliation between the United States and the Soviet Union.

His speech was phrased as a challenge to European countries and the United States to match the growing superiority of the Soviet arsenal. It was given Saturday, the start of a three-day conference sponsored by Georgetown University's Center for Strategic and International Studies and the Paris-based Atlantic Institute.

In questioning U.S. ability to defend Europe, Kissinger appeared certain in the short run to intensify current European doubts about U.S. intentions and leadership in NATO.

Highly placed British, West German, Belgian and French defense experts here said his speech could have a demoralizing effect on public opinion in their countries.

Most American participants, however, approved of the speech as a blunt message to European governments to plan more arms spending and a bigger European defense role to meet growing Soviet military might.