A comprehensive study prepared by top Soviet scientists in advance of the U.S.-Soviet arms talks charges that President Reagan's "Star Wars" plan is a dangerous and costly "deception" that could increase the chances of nuclear war.

The purpose of the 42-page, largely technical, report, which is dated only "1984," is unknown, but it apparently was prepared earlier last year to draw together the thinking of top Soviet scientists in several fields on the subject.

Although there is no indication that it played a role in preparation of Soviet policy for the talks beginning here Monday, it is the most comprehensive account of the Soviet estimate and attitude toward the Reagan administration's Strategic Defense Initiative known to have reached the West.

It also contains the first detailed list from Soviet officials of the "active" countermeasures the Soviets might use against Star Wars, including "space mines" and "clouds of obstacles" to be placed in space.

Two western reporters noticed a reference to the report in a recent Soviet technical journal, and one of them obtained it and shared it with the other.

Taken together with Washington statements, including those of Reagan last week, the report dramatizes the deep division between the two nuclear superpowers on a subject that is expected to loom large in the talks beginning here Monday between Secretary of State George P. Shultz and Foreign Minister Andrei Gromyko. A State Department spokesman with the Shultz party would not comment on the report.

The document concedes that parts of a space-based antiballistic missile system may be feasible at enormous cost, which it estimates at $1.5 trillion to $2 trillion.

But it maintains that any hoped-for increase in U.S. security would be illusory and charges that "the very attempt to create" such a system will be "a heavily destabilizing factor" in increasing the likelihood of a preemptive nuclear strike and all-out nuclear war.

The report was sponsored by what appears to be an ad hoc Committee of Soviet Scientists for Peace Against Nuclear Threat. It was prepared by a "working group" headed by the top Soviet space official, R.Z. Sagdeyev, director of the Institute of Space Research.

The report, which cites western and Soviet published materials, makes points and arguments that the Soviets are known to have an interest in making to the West, and there is no indication that it is a classified document.

Most of the document maintains a scholarly tone, including extensive mathematical and technical data designed to back up its case.

It departs notably from this posture, however, to denounce Reagan administration "assertions" that the Star Wars system "spells salvation from nuclear missiles for mankind." The report charges these are "perhaps the greatest-ever deceptions of our time."

Reagan said Thursday in a pamphlet released at the White House that "the United States seeks neither military superiority nor political advantage" in seeking to create a space-based antimissile defense. He said the effort is "both militarily and morally necessary" because the basic assumptions of security through the threat of nuclear retaliation are being called into question by the Soviet military buildup and improvements in defensive technology.

The Soviet researchers said that Reagan's plans, although originally set forth in a "very general and vague" manner in his March 1983 Star Wars address, "may mean a radical change" in the concept of nuclear deterrence through the threat of retaliatory strikes.

"The change of the official [U.S.] policy if it is consolidated, can create a fundamentally new strategic, political and psychological climate for decision-making on both offensive and defensive military programs."

The Soviet Union already has a relatively primitive "killer satellite" system intended to destroy intruding satellites, but this system lacks the speed and capability to attack missiles that advanced systems, including that envisioned by the United States, would seek.

In a section of the report devoted to possible countermeasures against a space-based antimissile program, the Soviet researchers listed five types of "active" countermeasures that could be used against Star Wars.

These include small, high-speed Soviet ballistic missiles that could destroy "combat stations in orbit," which the researchers anticipate some versions of the U.S. plan would need. Another countermeasure is "space mines," defined as satellites equipped with missiles and other weaponry. Another possibility is "clouds of obstacles" to be placed in space.

Ground-based lasers to attack the U.S. stations and decoy missile launchings to confuse the system also are discussed.

A "highly efficient countermeasure system" would cost only 1 to 2 percent of the cost of the Star Wars system, the report contended.

The report dismissed Reagan's argument that a large-scale antimissile defense system would bring new international stability. This argument, it continued, may have some "sense" if Reagan simultaneously would "renounce the buildup and improvement" of offensive nuclear forces.

The researchers said, however, that "what is happening is just the opposite" -- a buildup of strategic and tactical offensive weapons.

As a result, the report said, the creation of an antimissile system would complicate deterrence "because survival and damage limitation in a nuclear war would come to depend more heavily on carrying out" a preemptive strike.

This line of argument indicates that the Soviets believe the United States would be tempted to carry out a preemptive first strike against the Soviet Union while using its antimissile defenses to shield U.S. territory from a retaliatory Soviet strike.

At the same time, the report suggested, the Soviet Union may have a "greater incentive" to launch its own first-strike attack on the United States "while taking countermeasures against" a space-based antiballistic missile system.

In general, the report said, one should expect that ever newer and more complex weapons systems would be developed.

"While an antimissile system is being created and deployed," it said, "the strategic offensive weapons will be rapidly improving their means of penetrating this defense."

In particular, it said, deployment of ground- and sea-based strategic cruise missiles is likely to accelerate, making verification procedures even more difficult.

The document also focused on the likely impact of Reagan's plan on Soviet-American relations.

"Even if Soviet-American relations improve in the foreseeable future so much that the American side will be politically ready to conclude mutually acceptable and equitable agreements on the limitation and reduction of strategic weapons, the existence of the tested and deployed components of space-based antimissile systems even on a limited scale may substantially complicate the progress of the talks and reduce the chances for a timely Soviet-American understanding," it said.

The report restated Moscow's charge that even the testing of antimissile systems would "call into question" the 1972 Soviet-American treaty banning antiballistic missile (ABM) systems with the exception of one for each side. They quoted Article 5 of the treaty as saying "neither side shall develop, test or deploy ABM systems or components which are sea-based, air-based, space-based or mobile land-based."

The instability created by Reagan's plan, it said, would increase "the probability of making wrong decisions in a crisis situation."

Among the experts who participated in the preparation of the report were A.A. Kokoshkin, deputy director of the USA Institute, an official think tank that studies U.S. affairs, and space experts V.I. Shevchenko, V.G. Rodin, S.N. Rodionov, R.R. Nazirov and O.F. Prilutsky.