The Reagan administration headed into its first major foreign policy battle in Congress yesterday by formally proposing the sale of $8.5 billion worth of military equipment, including five sophisticated AWACS radar planes, to Saudi Arabia.

"This proposed sale is a cornerstone of the president's policy to strengthen the strategic environment of the Middle East," and President Reagan considers it essential to protecting vital U.S. interests, Undersecretary of State James L. Buckley said.

A majority of both houses of Congress have expressed opposition to the sale, which can be stopped if both houses reject it within a 50-day period that begins Sept. 9 when Congress returns from a recess.

Buckley took note of the vigorous Israeli opposition to the sale of AWACS (Airborne Warning and Control System) planes, but told reporters in a State Department auditorium: "Let me reaffirm that this administration remains committed to the security of Israel and will ensure that Israel maintains its substantial military advantage over potential adversaries. In short, we will not allow the regional balance of forces to be affected by the sale."

A State Department paper said the sale -- which also includes fuel tanks to add range to the Saudis' 62 F15 fighters, 1,177 Sidewinder air-to-air missiles and eight refueling aircraft -- "poses no significant threat to the security of Israel." It added that Israel has increased its air superiority over its Arab foes since the last Arab-Israel war in 1973.

The Israeli Embassy immediately issued a statement denouncing the proposed sale as "a danger to security and to the military balance in the area." Israel's objections are to the AWACS, not the other elements of the sale.

After reading a brief statement for reporters and television cameras, Buckley said that he and other officials would answer questions, but only on the condition that they not be identified other than as "senior administration officials."

Secretary of State Alexander M. Haig Jr. said Sunday that the administration intends to win what both sides predict will be a close battle in Congress over the sale. The senior administration officials who followed Buckley said they are confident that "the sheer logic" of the proposal will win it approval.

The administration argues that the sale serves U.S. national security interests by:

* Helping defend Saudi oilfields, which produce 63 percent of Persian Gulf oil.

* Restoring U.S. credibility with Saudi Arabia as a reliable security partner after a deterioration in U.S.-Saudi relations during the Carter administration.

* Providing a military system compatible with the United States' that would be in place to help respond to any Soviet-backed threat in the Persian Gulf. "Having such access in Saudi Arabia would therefore facilitate deployment of U.S. tactical air forces in the region in time of need, if so requested," an administration paper defending the sale says.

* Highlighting U.S. commitment to Saudi security and to the Reagan administration's policy of countering Soviet and Soviet-proxy threats in the region while laying a foundation for more extensive U.S.-Saudi military cooperation.

Saudi Arabia and the United States have agreed there will be restrictions on the Saudis' use of their AWACS, the senior officials said.

Saudi Arabia will be prohibited from allowing nationals of any third country to work with the radar planes, they said.

They were vague, however, about whether Saudi Arabia will be free to fly the AWACS anywhere over their own country, including northern Saudi Arabia, where the planes could monitor all Israeli aircraft movement.

One official said there would be some such restrictions, but another then refused to confirm that there would be any limits on Saudi use of AWACS in Saudi airspace.

The most senior of the senior officials also seemed to invite Israel to shoot down any AWACS that flew close to Israel: "For the Saudis to be that close to Israel would be very visible on Israeli detectors, radar and otherwise, and the longevity of such an aircraft would be particularly short-lived."

The United States is withholding from the five Saudi AWACS some of the most sophisticated electronic equipment aboard U.S. AWACS. The Saudi planes would not be equipped as well as the version of the AWACS being flown by the North Atlantic Treaty Organization, the officials said.

The sale establishes a new long-term U.S. presence in Saudi Arabia. About 400 civilian technicians and about 30 Air Force personnel will be on the ground with the AWACS when deliveries begin in mid-1985. The numbers will diminish, but some U.S. personnel will remain with the AWACS for the life of the planes.

At first, Americans will fly with Saudis, but these mixed crews gradually will be replaced by all-Saudi crews as they are trained.