A senior Soviet official warned today that the Soviet Union will adopt a policy of "automatic" massive retaliation against all potential enemies if new American medium-range nuclear missiles are deployed in Western Europe.

The statement by Anatoly Alexandrov, president of the Soviet Academy of Sciences, coincided with the resumption of Soviet-American talks in Geneva on limiting medium-range nuclear weapons in Europe and was the most explicit threat so far that Moscow would adopt a "launch on warning" posture to counter what he called a "terrible danger" posed by the new U.S. missiles.

Although the Soviets have frequently raised the possibility of using a "launch on warning" posture, Alexandrov's remarks were more precise on this issue. Some western observers here speculated, however, that they may be a bluff designed to frighten western public opinion. Speaking here at the opening of an international conference of scientists for nuclear disarmament, Alexandrov said the present balance of strategic forces allows roughly 30 minutes to both superpowers to "take some steps to avoid" a nuclear confrontation.

The planned deployment by the North Atlantic Treaty Organization of 572 Pershing II and cruise missiles in five Western European countries, which is to begin late this year, would reduce this "time span to only five to seven minutes" and thus "completely preclude" chances to avert an all-out confrontation, Alexandrov said.

"What remains is only automatic retaliation, with all available means, at all targets on the territories of all potential opponents," he said.

Alexandrov said the effects of a nuclear confrontation would be devastating for both sides, including large parts of the Soviet Union that would be "uninhabitable" for a long time, as well as a large part of Western Europe, if not the entire continent.

He said damage to large parts of the United States would be even greater because of the many atomic power plants, whose destruction would contribute devastation.

He expressed the hope, however, that antiwar forces throughout the world would exert pressures on the United States to "prevent a nuclear confrontation."

Alexandrov's remarks about Moscow's "automatic retaliation" were not reported by the government news agency Tass.

Under launch on warning, Soviet nuclear forces would be programmed for almost instant action against western targets if computerized Soviet intelligence monitoring facilities reported an imminent U.S. attack on the Soviet Union.

Alexandrov, a prominent physicist who worked on Soviet weapons systems including nuclear-powered naval vessels, is a senior figure. He was the only civilian to be called with top Soviet military commanders to attend a Kremlin meeting with the political leadership last October.

As head of the Academy of Sciences, he runs the scientific establishments with an annual budget greater than the official military budget. Much of the academy's money is for military research and development.

The conference also was addressed by a number of prominent Soviet figures including Boris Ponomarev, an alternate member of the Politburo, and Marshal S. F. Akhromeev, first deputy chief of the general staff.

Akhromeev told antiwar scientists from more than 20 countries that the Soviet military chiefs "do not see the possibility of winning a nuclear war." He assailed the U.S. military doctrine as having as its aim strategic domination over the Soviet Union.

The conference appeared to have been organized by Moscow to generate pressures in the West for a change in Reagan's military and foreign policies.