Prime Minister Menachem Begin, backed by a unanimous Cabinet, today reaffirmed Israel's "unreserved" opposition to the sale of surveillance aircraft and other enhanced weaponry to Saudi Arabia, but denied that Israel is interfering in U.S. congressional hearings on the $8.5 billion package.

In an apparent attempt to dispel any lingering notions in Washington that he might extricate President Reagan from his dilemma at the last minute by sending a signal to Congress that Israel could live with the Saudi deal, Begin said it was the "duty" of the government to voice its opposition

In a communique, the Israeli Cabinet "took note" of a report by Begin and other delegates to talks in Washington earlier this month that voiced "unreserved opposition to the sale of sophisticated weapons to Saudi Arabia, including offensive equipment for F15 planes and the most sophisticated intelligence planes, the AWACS," or airborne warning and control system.

Talking with reporters after the Cabinet meeting, Begin said there was "no foundation" to assertions by Sen. John Tower (R-Tex.) that Congress is, in effect, being asked to choose between Reagan and Begin.

Begin's remarks came as something of a surprise because the Israeli Foreign Ministry had taken a much lower profile on the AWACS package recently, reportedly instructing embassy officials in Washington to stop briefing members of Congress and the news media on Israel's grounds for opposing the sale. Government sources said it was felt that there already is sufficient opposition to kill the sale, and that further proselytizing would unnecessarily increase the tension in U.S.-Israeli relations.

Asked about Israel's lobbying effort, Begin replied, "I don't know if it is a high profile or a low profile. We did our duty . . . . On Oct. 20, there will be a vote taken on this issue in both houses of the Congress. We shall see the results."

Secretary of State Alexander M. Haig Jr., in a television interview for "Issues and Answers" (ABC, WJLA), said as many as 12 senators who signed a statement opposing the AWACS sale have said they may be willing to reconsider.

Begin insisted, in contrast to statements made by some U.S. officials, that in all his recent discussions in Washington he did not detect the "slightest hint" of any linkage between the AWACS package and proposed strategic cooperation between Israel and the United States.

There is a growing feeling among Begin's advisers that Israel has already sustained the bulk of any damage to its image that will result from the AWACS controversy, and that it has passed the point of no return. To even consider withdrawing its opposition to the Saudi deal would undermine Israel's strongest supporters in Congress, and would be costly to Begin's credibility at home.

The opposition Labor Party is as vehemently opposed to the AWACS sale as Begin's ruling Likud coalition, and Labor Party leader Shimon Peres has accused Begin of going soft on Israel's opposition in exchange for the strategic cooperation agreement.

The widespread perception in Israel, fueled by untiring press attention to the controversy, is that the computer-controlled airborne command capability of the AWACS, which can supervise a wide range of offensive ground and air operations simultaneously, would soon wipe out Israel's military superiority over Saudi Arabia and the rest of the Arab world.

Despite Begin's focus on the danger of Israel's armed forces becoming "transparent" to the AWACS, most officials say they regard the command function of the aircraft as more important to Israel than the radar-detection function.

Before Begin's trip to Washington, resignation to the inevitability of the sale was widespread among Begin's ruling circle and the emphasis was beginning to shift to the belief that Israel would have to demand that the United States supply it with developing technology and hardware to counter the Saudi AWACS.

Some Cabinet ministers were saying privately that continued opposition in a cause that was lost anyway would not be worth the risk of jeopardizing Israel's relations with the Reagan administration at a time when a record $3 billion aid request was being considered.

But with the emergence of the proposed strategic cooperation pact, coinciding with growing congressional support for Israel on the AWACS issue, the emphasis appears to be turning to an appreciation of U.S. dependence on Israel as a strategic necessity in the Middle East. supply it with developing technology and hardware to counter the Saudi AWACS.

Some Cabinet ministers were saying privately that continued opposition in a cause that was lost anyway would not be worth the risk of jeopardizing Israel's relations with the Reagan administration at a time when a record $3 billion aid request was being considered.

But with the emergence of the proposed strategic cooperation pact, coinciding with growing congressional support for Israel on the AWACS issue, the emphasis appears to be turning to an appreciation of U.S. dependence on Israel as a strategic necessity in the Middle East