If there were any doubts that the United States and the Soviet Union are now at one of their most critical junctures in the ambiguos relationship known as detente, they were swept away yesterday by presidential national security adviser Zbigniew Brzezinski.

An important shift of emphasis in the conduct of foreign policy also was publicly demonstrated yesterday, if President Carter chooses to leave the Brzezinski formulation of that policy unchallenged.

Should that occur, it would clearly signify that Brzezinski, and not Secretary of State Cyrus R. Vance, has unmistakbly emerged after 17 months as the dominant force in the competition to be principal foreign policy adviser to Carter.

As Brzezinski drew the line in the dust for the United States, the challenge he proclaimed most acutely is in Africa. He stretched beyond President Carter in public to add the Soviet Union, and East Germany, as well as Cuba and Angola, to the list of nations culpable in "a belligerent act" of force in Zaire.

The total Brzezinski indictment, however, was global.

In his appearance on "Meet the Press" (NBC, WRC), Brzezinksi portrayed a dynamic, threatening, worldwide Soviet challenge:

" . . . a sustained and massive effort to build up its conventional forces, particularly in Europe; to strengthen the concentration of its forces on the frontiers of China; to maintain a vitriolic worldwide propaganda campaign against the United States; to encircle and penetrate the Middle East; to stir up racial difficulties in Africa, and to make a mo ve difficult a moderate solution of these difficulties," and perhaps also "to seek more direct access to the Indain Ocean."

This represents, Brzezinski said, not a pattern of behavior "compatible with what was once called the code of detente," but rather a "shortsighted attempt to exploit global difficulties."

In some respects, it could be said that this is no departure from the attitude and convictions that Brzezinski has held since he entered the White House. With the concurrence of Carter, who openly described Brzezinski as his teacher in foreign policy, he has said from the outset that, in order to survive, U.S.-Soviet detente must be made more "reciprocal" - that detente is a two-way street.

But never before has any senior strategist in the Carter administration thrown down the gauntlet to the Soviet Union so startly, and at such a sensitive point.

Brzezinski was speaking yesterday fresh from the exhilaration of what he described, in an exclusive interview given the day before to The New York Times, as "a masterful analysis" of world affairs offered to him in Peking by Hua Kuo-feng, leader of the People's Republic of China, the Soviet Union's arch rival for communist supremacy in the world.

According to that report, Brzezinski and his aides discussed with Chinese officials the contests of secret White House memoranda on American worldwide security objectives, in the most extensive consultation of its kind since the United States and China reestablished diplomatic contact in 1971.

And Brzezinski, just returned from China, also was fresh from the opposite kind of encounter at the White House on Saturday morning between President Carter and Soviet Foreign Minister Andrei A. Gromyko. That meeting went very badly. The Soviet Union therefore needed to additional Brzezinski goading to stimulate its constantly high suspicions about what American strategists privately call the exploitation of "triangular" Washington-Peking-Moscow diplomacy.

The immediate reaction among Soviet sources in Washington was that Brzezinski had "delivered an ultimatum, as one source who declined use of his name put it.

That reaction was based on "the tonality" of Brzezinski in the television broadcast and, most of all, on the language he employed in describing the Carter administration's position on the central issue in Saturday's meetings with Gromyko. This is the continuing attempt to eliminate remaining obstacles in the four-year-old nuclear strategic arms limitation talks (SALT).

Brzezinski said yesterday that an agreement "is within grasp, if reason prevails." The United States, he said, has made "very proper, balanced proposals" to conclued a new treaty limiting intercontinental missiles and bombers.

"If they are accepted," Brzezinski said, "we could have agreement within days. If they are not accepted, we will wait until are accepted."

Brzezinski probably is in intend his language to sound as totally inflexible as it literally is in diplomatic terms. Inevitably, Russian sources who heard it immediately bristled, as one said, "if that is what the Carter administration actually means.

"That is not negotiating," said one Soviet source; "that is an ultimatum."

It should be added that the Soviet Union is by no means impartial where Brezinski is concerned. The Kremlin looks upon the Polish-born Brzezinski ax the arch devil in the Carter administration; he has always been a special target of Soviet attack. "But this is much worse than we ever heard from Brzezinski," said one Soviet source; "this is very, very stiff - even from Brzezinski."

What now will preoccupy the leadership in the Kremlin - as it will the senior officialdom of the Carter administration - is whether the Brzezinski presentation is the controlling policy of the United States and, if it is controlling, what the United States will actually do to try sustain it.

From all indications in the White House, Brzezinski is far closer to Carter's inclinations at this stage than are soft-spoken Cy Vance, the entire hierarchy of the State Department, the Arms Control and Disarmament Agency, Africa "point man" Andrew Young and his ambassadorial staff at the United Nations, and all the other advocates of non-strident policy inside the administration.

A national stiffening of attitude toward the Soviet Union is under way in this country, among the public, in Congress, inflamed by Soviet-Cuban actions in Africa amd prosecution of Soviet advocates of human rights. Former president Ford and his secretary of state, Henry A. Kissinger, are now in the vanguard of those demanding that the line be drawn against what Brzezinski in Peking labeled Soviet "international marauders."

President Carter has displayed rising responsiveness to this sense of outrage in his own public comments.Another opportunity will come at the 15-nation North Atlantic Treaty Organization summit conference which opens here tomorrow.

Carter's meeting Saturday with Gromyko is known to have drawn the president farther in the direction that Brzezinski marked out yesterday. The fact that Brzezinski on national television gave the first extended commentary on the state of U.S.-Soviet relations immediately after the Carter-Gromyko talks appears to carry its own message. The broadcast was prerecorded Saturday just three hours after the Gromyko talks.

After the four-hour discussion in the White House on Saturday, indignation arose over Gromyko's challenge - raise inside the meeting, and afterward to the press outside - to the accuracy of the president's accusations about the Soviet-Cuban role in the recent raid from Angola into Zaire.

"Gromyko lied," one administration official said with exasperation and puzzlement, "when he didn't have to lie. Why did he lie in the White House?"

As his official, and others in the White House saw it, Gromyko, as a veteran diplomat, had numerous deflecting options open beyond overtly challenging the intelligence data that Carter confronted him with.

One source recalled solemnly, "It was Gromyko who lied to Kennedy about the missiles in Cuba" at the outset of the 1962 Cuban nuclear missile crisis. In that incomparably grimmer confrontation, the United States and the Soviet Union dangled on the edge of World War III over the secret emplacement in Cuba of SOviet missiles with range enough to attack the United States.

Gromyko's counter-claim Saturday, as he expressed it later upon leaving the State Department, was that "there is information - and information. And sometimes conclusions are drawn from incorrect and inexact information. And that is bad."

A Soviet source indignantly said yesterday that "American intelligence knows for sure that we haven't made this Zaire thing. They know we didn't start it."

Brzezinski himself yesterday supplied what may be at least part of the explanation for the totally divergent American and Soviet versions of Russian complicity in the military action in Zaire.

When asked if he was charging the communist nations with "direct participation" in fighting, commanding or controlling the raid into Zaire by ex-Katanga gendarmes, as they are called, Brzezinski made no such direct claim. He said, "We are talking about responsibility . . . for something which should have not taken place . . ."

By these standards, even Soviet weaponry in the hands of the rebels who crossed into Zaire might be construed as a measure of Soviet "responsibility." The Soviet Union supplies weapons to guerrilla forces across Africa. The U.S. charges presumably go beyond that.

Beyond the facts in dispute, however, there is a fundamental difference in attitude between many Carter political insiders and many veteran diplomats about what is happening right now.

If Gromyko in fact was untruthful to Carter, that comes as no great shock to experienced diplomats. "Every country lies when it has to - including us - to protect its national interests," said one veteran. "That should come as no surprise anyone dealing in realities."

To those most familiar with Soviet behavior, it was inconceivable to expect Gromyko to do anything but flatly deny Cuban, Angolan, East German and especially Soviet complicity in the attack on Zaire - regardless of the facts. Cuba and Angola, which are Soviet allies, are repeatedly on record denying any involvement in the cross-border action, direct or indirect. It was inescapable that Gromyko would agree with them.

From a Soviet perception, Gromyko otherwise went far - by Kremlin standards - to avoid exacerbating U.S.-Soviet relations during the past week. Senior State Department officials said last Wednesday that they expected Gromyko to respond with at least equal harshness to Vice President Mondale's attack that day, before the United Nations, on Soviet military expansionism. Next day, it was Carter himself, in Chicago, who warned that Soviet-Cuban actions in Africa could endanger conclusion of a SALT pact.

Gromyko, nevertheless, was milder than American strategist anticiapated in his own speech to the United Nations on Friday, and Vance told the Senate Foreign Relations Committee the same day that prospects looked good for overcoming the remaining differences on SALT.

By Saturday, all bets were off. And by 1 p.m. yesterday, when Brzezinski finished speaking, U.S.-Soviet relations as a whole were at a new level of anxiety.