Soviet Foreign Minister Andrei Gromyko today flatly rejected as "impossible" any attempt to renegotiate the SALT II treaty if it is rejected or altered by the U.S. Senate.

Such Senate action, he said, would mean "the end of negotiations" since "those bridges" established to open the third phase in U. S.-Soviet strategic arms limitation talks "would be destroyed."

Gromyko's statements at a news conference echoed and reinforced the warning Soviet President Leonid Brezhenev made during a dinner with President Carter on June 17. Any attempt to revise the treaty, Brezhnev said in his toast, could lead to "grave and even dangerous consequences" for Soviet -American relations and the whole world.

But while Brezhnev implied that Moscow would not reopen talks to discuss possible Senate revisions, Gromyko today explicitly ruled out renegotiations.

"I tell you frankly, it is impossible to resume negotiations," Gromyko said.

"It will be the end of negotiations . . . the end . . . No matter what amendments would be made, it would be impossible on the basis of these amendments to open negotiations . . . a fantastic situation," Gromyko asserted vigorously in English in response to a question from an American correspondnet.

If the treaty is not ratified by the Senate, Gromyko said, "The situation will be very complex, very bad. There would not be that curbing of the arms race provided for in the treaty - the curbing and limitation. And those bridges which lead from the treaty signed in Vienna to a possible SALT III would be destroyed. Those politicians who have not yet decided their stand, and those who have decided but in the wrong direction, should think about that, too. Justice and objectivity should rule."

Ostensibly a report to the Soviet people about the Vienna summit, much of Gromyko's press conference clearly was aimed at the lawmaking chamber 5,000 miles away in Washington where the treaty is under sharp attack and the administration apparently is well short of the 67 votes needed for ratification.

[In Washington, Senate Majority Leader Robert Byrd (D. W. Va.) said the Senate "will not be guided or influence" by Soviet statements. "This is not the time for inflammatory rhetoric and I would hope that this debate can be as free as possible from early hardline posturing on either side," he said.]

Technically the Senate cannot amend the treaty. Rather it could refuse its consent unless certain portions are renegotiated. But short of that, the Senate could vote "reservations", which represent U. S. interpretations of treaty language; and it could approve "sense-of-the-Senate reolutions," which could direct the U. S. government to take certain actions. Neither the reservations nor sense of the Senate resolutions would have any direct effect on the Soviet Union.

The well-known Jackson amendment to the SALT I treaty was in fact a sense-of-the-Senate resolution that said that in future negotiations, the U. S. would have to have equal numbers of strategic weapons on both sides, a directive that was followed in SALT II.

In the most authoritative Kremlin assessment to date of the four-day summit, Gromyko underscored Moscow's view that the treaty is now inviolate and supremely important to Soviet-American relations in the coming decade.

"I can be frank about it," said the solemn-faced diplomat who has been foreign minister for 22 years. "This meeting was filled with great political content, of exceptional importance, maybe even more than any other international meeting of its sort."

He argued that the nearly seven years of negotiations "was a long-enough period to weigh . . . several times . . . even comparatively small conclusions relating to strategic weapons, in order to avoid miscalculation. The result is a treaty acceptable to both sides."

He stressed that the Soviet Union wants further bilateral strategic arms control efforts, which hinge on approval of SALT II. "A crucial time is beginning now in the United States when after the treaty is signed, certain quarters are still weighing up which position to take. I would like to express the hope that this will be a correct and objective position.

"The treaty lays the foundation for further advance, for the preparation of a new agreement on further strategic arms limitations. And in theis lies its tremendous value.

"A joint statement has been published devoted to future negotiations on the possible conclusion of a new SALT treaty. Thus, the present treaty throws bridges over to the next treaty.

"In the final analysis, we must strive for the first curtailment to be followed by another, and the other to be followed by a third curtailment, and so on. This is the beginning of a process which for its gravity can hardly be compared to any other process."

In an hour-long question period after his one-hour statement in an auditorium of a swank Lenin Hills residence for officeal guests, Gromkyo asserted that the treaty is verifiable by the United States. Treaty opponents in the United States contend that because of telemetry coding and loss of U.S. listening posts in Iran, Soviet tests of new missiles cannot be adequately verified.

"If politicians in the U.S. don't agree, I'm sorry." Gromyko said, and recommended they get reassurance on the issue from "their national agencies," an obvious reference to American intelligence-gathering agencies. He added that Carter and the entire U.S. summit side "expressed the unanimous opinion" during the talks that SALT II is verifiable now.

He said the Soviets have never failed to live up to the terms of any international treaty they signed. Gromyko was one of four Politburo members on the Soviet side at Vienna.

He twice emphasized that the ailing Brezhnev "personally" oversaw the long negotiations, a reference apparently to blunt Western news dispatches beamed into the Soviet Union from abroad that described the Soviet leader's evident frailty in Vienna.

"I wish to specially stress this," said Gromyko. "Leonid Brezhnev, who knows the problem well, constantly watched over the main questions and lines of the negotiations."

On other matters discussed at the summit, Gromyko said the Soviets rejected U. S. attempts to gain Kremlin backing for the peace treaty between Israel and Egypt. He quoted Brezhnev as saying that "there can be no question of the Soviet Union's support for the anti-Arab treaty as well as for any mechanism which is being created to service the treaty's implementation."

"The Soviet position remains as it has for many years," he said. "All Arab land captured by Israel must be returned. The Palestinians must have the opportuntiy to set up a small, independent state."

He recalled that the Kremlin was an original supporter of the Israeli state, something Soviet propagandists now virtually never mention. "No one should have any doubt about Israel's right to exist," Gromyko said. "Israel should appreciate the Soviet position." CAPTION: Picture, ANDREI GROMYKO . . . renegotiation is "impossible"