In a sharp reply to advocates of a nuclear weapons freeze and other shifts in U.S. arms strategy, Secretary of State Alexander M. Haig Jr. warned yesterday that "the stakes are too great and the consequences of error too catastrophic" to scrap time-tested policies "for a leap into the unknown."

A freeze on the buildup of both Soviet and American nuclear weapons as a step towards halting the arms race and reducing the risk of atomic war is being urged by a group of senators and within a number of communities in this country and in Western Europe.

In a major address to Georgetown University's Center for Strategic and International Studies, Haig said that the administration is "sensitive to the concerns underlying this proposal."

But he stoutly defended the administration view that the safest and best way to deter a nuclear attack and ultimately to reduce the weapons on each side is to "maintain the military balance now being threatened by the Soviet buildup."

State Department officials said that Haig's speech was an attempt both to "head off" the freeze movement in this country and to launch a preemptive strike of his own at a proposal about to be unveiled in a new issue of the magazine Foreign Affairs. That proposal calls for the element of its force structure--its heavy, multi-warheaded intercontinental missiles--unless it is persuaded that otherwise the United States will respond by deploying comparable systems itself," such as the new MX missile.

In his effort to counter those who want the United States to renounce the "first use" of nuclear weapons in Europe, Haig noted that the Soviets have frequently proposed a pledge against "no first use" but NATO "has consistently rejected" it.

Because NATO is outnumbered and outgunned in conventional troops and armor by Soviet-led Warsaw Pact forces, Haig claimed such a proposal would be "tantamount to making Europe safe for conventional aggression." He said such a policy would force the West to maintain the same huge levels of ground forces as the Communist bloc and that those who advocate "no first use" seldom go on to propose "reintroduction of the draft, tripling the armed forces, and putting the economy on a wartime footing."

NATO, as part of the flexible response strategy adopted in 1967, has had a plan in which, if Europe were being overrun by Soviet conventional forces, it might resort to nuclear weapons to try to stop the onslaught. Haig said that flexible response is not premised on the view that nuclear war can be controlled. He added, "Every successive allied and American government has been convinced that a nuclear war, once initiated, could escape control" and that is why the possible nuclear response in Europe is not "in any sense automatic."

Nevertheless, he argued, "nuclear deterrence and collective defense have preserved peace in Europe" and "prevented a conflict between the two superpowers."

Haig's speech, which was touted the day before by President Reagan as an important statement of administration views, also dwelled on the psychology of deterrence, which, he said, must convince a potential aggressor that "he cannot prevail in any conflict with us."

"Deterrence faces its true test at the time of maximum tension, even in the midst of actual conflict," Haig said. "In such extreme circumstances, when the stakes on the table may already be immense, when Soviet leaders may feel the very existence of their regime is threatened, who can say whether or not they would run massive risks if they believed that in the end the Soviet state would prevail."

Haig argued that "every judgment we make and every judgment the Soviet leadership makes will be shaded" by the state of the strategic balance between the two countries. Thus Moscow, "in calculating the risks of subversion or aggression . . . or nuclear blackmail or political intimidation of our friends," must evaluate the possibilities of an effective U.S. response and also "American willingness to face the prospect of U.S.-Soviet confrontation and consequent escalation."

Deterrence requires, therefore, "maintenance of a secure military balance, one which cannot be overturned through surprise attack or sudden technological breakthrough." It is, consequently, "more than military strategy."

"It is the essential political bargain which binds together the western coalition," he said.