Calling it "a giant step" toward arms reduction, President Carter's national security adviser yesterday revealed new details of the U.S. strategic arms limitation proposal, which the Soviet Union has bluntly rejected.

Zbigniew Brzezinski, speaking at an unusual White House news conference, held out hope that the Soviets eventually will accept the essence of the U.S. plan despite their vehement initial reaction.

"We made the damnedest effort to produce a package which . . . as reasonably equitable for both sides," said Brzezinski. His words, his cool demeanor and the news conference were in rebuttal to Soviet Foreign Minister Andrei Gromyko's denunciation of the plan as a "cheap and shady maneuver" designed for one-sided U.S. advantage.

The presidential assistant said nobody in the White House expected the Soviet Union to accept the U.S. proposal during the three days of Secretary of State Cyrus R. Vance's Moscow visit, but added that "we expected them to consider it."

Brzezinski made no claim that the United States had anticipated the extremely strong Soviet reaction, including Gromyko's bitter comments. He suggested that Soviet defensiveness in being outflanked as a disarmament advocate was an element in Gromyko's reaction.

Condemning the U.S. Soviet Vladivostok agreement of November, 1974, as an arrangement for continued arms competition, Brzezinski described the Carter plan as "in many respects the first truly, genuinely disarmament-oriented proposal introduced into the Strategic Arms Limitation Talks."

As he explained it, the essential U.S. aims were to freeze and in cases reduce the numbers of nuclear-armed strategic weapons aimed at one another, and to halt the race for improved destructive power by banning technological improvements. Gromyko, in his Moscow news conference, interpreted both these aims as efforts of the United States to seek advantage.

Brzezinski said the U.S. plan called for:

Reduction of total strategic missiles or bombers allowed each side from 2,400 each (the Vladivostok level) to 1,800 to 2,000 each.

Reduction of multiple warhead (MIRV) launchers from 1,320 each (the Vladivostok level) to between 1,100 and 1,200 each.

A limit on land-based, multiple warhead intercontinental ballistic missiles (ICBMs) to 550 each. (There was no such limit before.)

Reduction of Soviet "modern large ballistic missiles" from the 308 deployed by the Soviets to a new upper limit of 150. This proposal to "deprive the Soviet Union" of half of these very big missiles was singled out for special attack by Gromyko.

A freeze on deployment of all ICBMs, a ban on modification of ICBMs and a limit on the number of annual flight tests.

A ban on development, testing and deployment of new types of ICBMs, particularly mobile ones.

An unspecified "arrangement" to assure the United States that the Soviet Backfire bomber would not be deployed as a strategic weapon.

A ban on "strategic" cruise missiles, which was defined as those capable of "transcontinental operation" or having a range in excess of weapons typically considered to be strategic.

The ban on futura technological improvement of strategic weapons, which was not attempted in earlier SALT agreements, drew fire from Gromyko as tending to "put the Soviet Union into a worse position than the United States."

But Brzezinski defended this as necessary for the long-range stability of an agreement, saying that otherwise "what may seem stable in 1977 or 1978 could become very unstable in 1980 or 1985."

Currently, the United States has 2, 128 strategic weapons deployed and the Soviet Union 2,540. The U.S. total is comprised of 1,054 land-based missiles, 656 submarine missiles and 418 long-range bombers. The Soviet strategic force consists of 1,450 ICBMs, 880 submarine missiles and 210 bombers.

Under the Carter proposal, the United States could keep its present arsenal nearly intact, while the Soviets would have to scrap hundreds of strategic weapons. The Carter ceiling of 1,800 delivery systems would require the United States to retire 328 strategic weapons and the Soviet Union 740, or almost twice as many.

Brzezinski said the United States presented the ertensive plan directly to top Soviet leaders, rather than sending it first through diplomatic channels, in an effort to circumvent the Soviet military bureaucracy. Under normal conditions an arms control proposal would reach Soviet leaders with "a categorical critique" supplied by their defense ministry, he said. "We wanted the top Soviet leaders $99[Word illegible]

Brzezinski suggested hopefully that unfolding events will follow the pattern of the late 1960s, when the Soviet Union initially repected a U.S. proposal to stop the deployment of antiballistic missiles (ABM) by both sides.

Referring twice to this analogy, Brzezinski said the anti-ABM argument was "not convincing" to Soviet Premier Alexei Kosygin when presented in 1968 because it was new to him. The Russians finally accepted sharp limits on ABMs in 1972.

Brzezinski declared that the far reaching U.S. proposal was designed with political as well as strategic considerations in mind, and several times referred to the hope that more cooperative, stable and accommodating U.S. Soviet relations would result from fair and stable strategic arrangements.

He said he was encouraged by Soviet willingness to establish working groups on other aspects of arms control, but otherwise made no attempt to suggest that the U.S. plan, so far, had encouraged the closer political relationship which is desired.