The United States and the Soviet Union have reached basic agreement on the long-awaited SALT II pact to restrain the nuclear arms race, the Carter administration announced yesterday.

Secretary of State Cyrus R. Vance, who headed the U.S. team during protracted and often difficult negotiations, made the announcement before reporters and cameras at the White House. Secretary of Defense Harold Brown, who is to take a leading role in selling the treaty to the Senate and the public, also made a statement.

President Carter and Soviet President Leonid I. Brezhnev are expected to sign the new strategic arms limitation treaty at a summit meeting in Europe next month. The time and place of the meeting are likely to be agreed upon and made public before the end of this week, White House sources said.

There was no immediate announcement in Moscow regarding agreement on the basic treaty terms. Final draftsmanship of the 70-page treaty and resolution of what were described as "a few remaining secondary items" are to be completed by the semi-permanent SALT delegations of the two sides in Geneva in the next several weeks.

Vance noted that the agreement on the basic treaty terms "marks the end of one negotiation, but it will open the way for another"-the struggle to obtain approval by the required two-thirds of the U.S. Senate.

The national debate, involving conflicting views of relations with the Soviet Union, as well as the complex and sometimes stupefying facts of the vast atomic weaponry on both sides, is expected to generate one of the most intense and important legislative battles of the decade.

Initial Senate reaction ran the gamut from strong support to strong opposition, with several key lawmakers reserving judgment. Some Democratic leaders predicted that the treaty will be approved, while Republican leaders suggested that it will be amended or returned for further negotiations.

The president opened the new phase of the campaign for what he has described as his most important foreign policy initiative. He presented his case for the treaty to Senate Majority Leader Robert C. Byrd (D-W.V.) at 7 a.m. and to Senate Minority Leader Howard H. Baker Jr. (R-Tenn.) at 11 a.m.

In mid-afternoon Carter briefed about 20 members of key Senate committees and last night repeated to the Democratic congressional campaign dinner his case that SALT II "will lessen the danger of nuclear destruction, while safeguarding our military security." Carter also sent letters backing the treaty to all 100 senators yesterday.

Vance was chosen to make the public announcement because he has carried the brunt of the long negotiation, according to White House press secretary Jody Powell, and because Vance and Brown are expected to lead the fight, in Senate testimony, for approval.

The Senate Foreign Relations Committee tentatively scheduled hearings beginning June 25 to consider the treaty. The issue is expected to reach the Senate floor in the fall.

Vance met with Soviet Ambassador Anatoliy F. Dobrynin agains for 50 minutes late yesterday for further discussions of the summit. It was their fourth meeting in three days, and their 27th meeting since Jan. 1.

The Vance-Dobrynin meetings this year were the forum for the final phase of the SALT II negotiations, which began 6 1/2 years ago in Geneva and came close to completion several times in the Ford and Carter administrations.

Vance called the treaty "an essential step toward a safer America and a safer world." He said its main accomplishments were equal ceilings on U.S. and Soviet strategic forces, a beginning on actual reductions in the level of nuclear arms and the first limitations on the qualitative race in nuclear weaponry.

Brown, addressing a critical point in the political debate, said, "We will be able to detect any Soviet violation in ample time to protect our military security." He said that without the treaty, the United States could be faced with "concealment, countermeasures and cheating of all sorts," because the Soviet Union would be free of restraints written into the document.

After hearing the case for the treaty, senators expressed a wide range of reactions. Majority Whip Alan Cranston (D-Calif.) said, "I think it will pass." Edward M. Kennedy (D-Mass.) expressed support, and said, "The burden will be on those who oppose it to show how the national security of the United States will be enhanced by defeating this treaty."

John C. Stennis (D-Miss.), chairman of the Armed Services Committee, said, "I have to be convinced . . . I am not committed." He expressed reservations on the matter of surveillance of Soviet military programs.

John H. Glenn (D-Ohio), a key figure on the verification issue, said it is "still a gamble" and "not yet adequately answered."

S.I. Hayakawa (R-Calif.), a conservative, said he had been impressed with Carter's description of the political consequences of rejection. "It would give the Soviets a tremendous propagada victory . . . as the defenders of peace and we as the warmongers," the uncommitted senator said.

Baker said he is "leaning against" the treaty, and that he has "a strong feeling" that the Senate will amend it or return it to the president for further negotiations.

Henry M. Jackson (D-Wash.) called the treaty "substantially unequal and unverifiable," and said it favors the Soviets. However, he suggested that the Senate will amend it to remedy shortcomings and "plug its many loop-holes."

John G. Tower (R-Tex.) said none of his doubts had been resolved and that "I will have to work for, and vote for, its rejection."

In background briefings for reporters, officials shed little light on the last-minute issues that have held up completion of the treaty in the past several weeks.

Other sources identified the issues as highly technical questions involving the ability of either side to launch multiple independently targetable reentry vehicle (MIR V) warheads than are permitted on a single missile under the terms of the proposed treaty.

Soviet tests last December showed a capability to launch 12 to 14 warheads on an SS18 missile, which would be limited to 10 warheads under the treaty.

In last-minute bargaining, the United States asked the Soviets to devise and observe procedures by which U.S. intelligence could distinguish between the 10 warheads permitted by SALT II, on the one hand, and the launch of dummy warheads or chaff above the permitted limit, on the other hand.

The Soviets, in reply, demanded that the United States take measures against the launch of extra warheads, above the permitted limits, on U.S. weapon systems.

After some hesitation, both sides agreed to take steps to meet the objections of the other, informed sources said.

Carter and the other administration officials made the point that several U.S. strategic weapons programs will continue outside the restraints of SALT II. Brown projected that "moderately increased programs" of U.S. weaponry will go forward under SALT II, and background briefers spelled out planned new land-based and submarine-based missile systems.

A briefing official also said that the United States reserves the right to deploy a class of bomber similar to the Soviet Backfire, which is not counted as a strategic weapon under SALT II but is subject to some restraints furnished separately by the Soviet Union.

The assurances on the Backfire are to be furnished at the time the treaty is signed at the Carter-Brezhnev summit meeting.

High officials have said that they expect a review of SALT II along with discussion of further reductions in SALT III when Carter and Brezhnev meet. Carter has been quoted several times as saying he is prepared to negotiate greater restraints on the spot at a summit if Brezhnev will go along. However, Brezhnev's deteriorating health has raised increasing doubt that the first U.S.-Soviet summit meeting of the Carter administration will be able to go much beyond formalities.

In addition to bilateral issues such as trade and cultural exchanges, U.S. officials are hoping for a full exchange of views with the Soviets on a wide range of contentious international problems in the Middle East, Africa, Asia and Cuba where the two super-powers are at cross purposes. Some officials suggest the United States would like specific and detailed discussions of such things, but it is questionable whether Brezhnev's health will permit such talk. CAPTION: Picture 1, Secretary of State Vance and Defense Secretary Brown brief reporters on treaty. AP; Picture 2, Soviet Ambassador Dobrynin meets with Secretary of State Vance after the announcement of the SALT agreement. UPI