Soviet President Leonid Brezhnev said tonight that any attempt to change the provisions of SALT II, which is to be signed here Monday, could lead to "grave and even dangerous consequences" for Soviet-American relations and the whole world.

Brezhnev's statement, by far his strongest note of warning against U.S. Senate attempts to amend the strategic arms limitation treaty, was contained in a toast to be delivered at a working dinner with President Carter.

[In Washington, Sen. Henry M. Jackson (D-Wash.) said he would fight in the Senate to send the arms pact back to the bargaining table. story, Page A17.]

The U.S. president, in his toast, warned that any Soviet attempt to exploit turbulence in Africa, Asia and the Middle East could bring about a superpower confrontation in which the United States would "protect its vita interests,"

Despite the tough public rhetoric, U.S. and Soviet spokesmen insisted that the past two days of private meetings have been a good beginning toward better understanding and rapport.

"It has been a realistic and sober meeting . . . a summit neither of confrontation nor euphoria," said a high U.S. official reporting on the carter Brezhnev discussions.

Go give substances to the talk of better communications, even if there is no agreement on many issues, the defense ministers and uniformed chiefs of general staffs of the two countries of general staffs of the two countries met for an hour and 40 minutes this afternoon. It was the first military meeting on this level between the two countries since the days of the World War 11 alliance.

Soviet reaction to domestic politics in the United state, more than any irritation with Carter, appeared to be the cause of Brezhnev's tough words about changes in the treaty that the two men are to sign in a restored imperial palace at midday Monday.

Earlier today, Carter gave a helping hand when the ailing Soviet leader stumbled and almost fell coming down a few steps after one of their meetings. With his statement tonight, Brezhnev, by his own reckoning, may have been seeking to lend a helping hand to Carter in the U.S. ratification fight.

Saying that it is "a fair balance of interests" which involved compromised on both sides, brezhnev said "every provision . . . every word" of SALT II had been weighed and pondered scores of times in the nearly seven years of U.S. Soviet netotitaions.

"any attmeptto rock this elaborate structure which it has been so hard to build, to substitute any of its elements, to pull it closer to one's own self would be an unprofitable exercise.

"The entire structure might then collapse, entailing grave and even dangerous consequences for our relations and for the situation in the world as a whole brezhnev said.

An another point he said the Soviet Communist Party, government and people will fully support the SALT II treaty as written but "will not accept any backpedaling, any attempt to undermine its spirit and its letter."

Carter and high administration aides have expressed the belief for many weeks that the Soviets would be unlikely to accept any substantive Senate amendments to the painfully negotiatied treaty. Brezhnevhs words seemed to back up this conclusion forcefully.

Carter's tough talk about regional confrontations followed a meeting of nearly two hours with brezhnev on the subject of political disputes around the world.

In this connection, remarks by Brezhnev to Carter and by Soviet spokesman Leonid Zamyatin to reporters seemed to hint that the Soviets, despite U.S. pleas, will cast their Security Council veto to stop a United Nations force from policing the Israeli withdrawal in the Sinai desert. The withdrawal is called for by the U.S. sponsored Egyptian-israeli peace treaty.

"The U.S.S.R. will not support proposals to use the United Nations in bolstering this seperate deal between Egypt and Israel," said Samyatin. U.S. officials confirmed that Brezhnev had said much the same to Carter. But the officials said they did not take this as a clear signal from Moscow that the veto would be cast.

In their meeting on regional disputes, Carter named the Horn of Africa, Afghanistan, southern Africa, Cuba, the Middle East and Southeast Asia as "turbulent" areas where U.S. concerns over Soviet involvement run high.

Carter appealed in his toast tonight for the Soviet Union's ally, Vietnam, to withdraw its troops from neighboring Cambodia and asked for compassion for tens of thousands of Indochina refugees.

Spokesman Zamyatin expressed little sympathy, charging that the Chinese-backed Pol Pot government in Cambodia had annihilated 3 million people, and that the refugees who are fleeing Cambodia now are Chinese.

The issue of Soviet naval vessels using the Vietnamese port of Cam Ranh Bay was raised by Carter in the talks with Brezhnev, according to American officials.

The officials said that Brezhnev raised the U.S. military alliance with South Korea.

No resolution of the discussion in either of these cases was reported.

The Carter summit team did not seem upset by the open discord with the Soviets on a variety of political and regional issues, and some officials seemed to relish the idea. But their descriptions, Carter is capable of working cooperatively with the Soviets to reduce world tensions on the level of SALT II while being outspoken and tough on other issues.

"Straighforward talk is necessary and helpful," a White House official said tonight. "Even with frank exchanges, the overall nature [of the Carter-Brezhnve talks] was increasingly positive."

Carter and Brezhnev are to begin their final day of this summit meeting with private talks at the U.S. Embassy followed by another meeting of the two delegations. The two leaders will then go to the Redoutensaal for the signing ceremony of SALT II, after which Carter leaves immediately for Washington. The president is scheduled to arrive at Andrews Air Force Base at 7:10 p.m. Monday and to address a joint session of Congress at 9 p.m. CAPTION:

Picture 1, Carter holds Brezhnev's hand as the two leaders leave meeting at Soviet Embassy. UPI; Picture 2, Presidents Brezhnev, left, and Carter meet at the Soviet Embassy in Vienna. AP