The Soviet Union charged today that President Reagan's strategic modernization plan would give the United States the capability to launch a first strike against the Soviet Union and thus "signals the start of a qualitatively new stage in the nuclear arms race." t

If the plan is implemented, the official news agency Tass said, the Soviet Union would have to start its own programs to develop "an appropriate counterbalance" to the weapons systems contemplated by the United States.

At the heart of Soviet concern seems to be Reagan's decision to go ahead with building of 100 MX missiles, which Tass described as a "fundamentally new generation weapon designed not for deterrence but for launching the first strike and waging a nuclear war."

The commentary also challenged Reagan's sincerity about the proposed Soviet-American arms limitation talks in Geneva, which are scheduled to begin Nov. 30. It said that "a limitation of arms is by far more important for ensuring the security of the United States than their buildup."

The fact that Reagan "noted only in passing" the prospective Geneva talks while announcing a "gigantic" arms buildup "calls in question the seriousness" of his administration to conduct talks on the basis of "equality and equal security" for both sides.

It said, "Reagan's verbiage about the necessity to restore the mythical safety margin and eliminate the so-called window of vulnerability is no more than a propaganda smokescreen to cover the hegemonistic plans of the United States."

This assertion, the commentary said, should be judged against the statements about the existence of rough parity made by presidents Nixon, Ford and Carter.

The commentary said that the U.S. arms buildup would not provide greater security to the United States but exposes it and "the entire world" to the "fatal danger" of a nuclear war.

It specifically challenged Reagan's premise that the planned buildup would give the United states a greater flexibility in meeting Soviet challenges in various parts of the world. Any use of such weapons envisioned in the so-called limited nuclear engagement would not bypass America, the commentary said.

Any use of nuclear weapons in a regional theater holds the possibility of electro-magnetic disturbances in the atmosphere that could block all communications and control facilkities and render an enemy virtually defenseless. As a result of this and other considerations, Soviet strategists hold that any use of such weapons near the Soviet territory would have to be met by an immediate, all-out retaliatory strike.

The tone of the Tass commentary was factual and restrained. It did not give the full details about the scope of Reagan's proposals. Its main conclusion, however, was that the plan was designed to give the United States "the capability to launch a first strike."

This conclusionis based on the "character" of the American triad -- land-based and sea-based nuclear missiles and strategic bombers -- and "the emphasis on accuracy, reliability and maneuverability of the new weapons."

In the arcane world of nuclear weaponry, each side has sought to have sufficient forces to discourage the other from attacking first by maintaining enough power to destroy the first strike force before it is launched. Any arms control limitation effort requires basic parity in these forces to ensure success. The drive for first-strike capability by one means the other would also seek to acquire its own first strike.

Tass tonight restated President Leonid I. Brezhnev's assertion that the Soviet Union is not seeking military superiority and wants only "reliable protection" for itself and its allies.

"But the Soviet Union will not be impartial to the appearance in the U.S. arsenal of new, even more terrible types of weapons. If this happens, the Soviet armed forces will have an appropriate counterbalance to such weapons."

Another Tass dispatch charged that the Reagan administration is developing a "multilayer system of antibalistic missiles [ABMs]" in an effort to achieve "complete invulnerability" for its first strike force.

The agency said that such a system "would violate the Soviet-American treaty on limitation of ABM systems which established ceilings on the deployment" of such weapons.

A separate Tass dispatch from Washington said Reagan's program "deals a blow to efforts to fulfill existing and future agreements in the area of arms control. It envisages the production of strategic arms systems which reduce to zero the chances of their control."

The MX, coupled with Reagan's decision to continue building Trident nuclear submarine and develop a new sea-launched missile, "will result in a sharp escalation of the arms race," the dispatch said.