Israel declared "unreserved opposition" today to President Reagan's plans to press ahead with the sale of surveillance aircraft and offensive weaponry to Suadi Arabia.

The Israelis warned that it will undermine peace in the Middle East and threaten the security of Israel.

In the strongest statement yet on the planned sale, Prime Minister Menachem Begin's office voiced "profound regret" at the White House decision, saying that it will create a "grave danger" to Israel in the future. dBegin repeated the warning in a meeting in his office with the U.S. ambassador to Israel, Samuel Lewis.

[In Washington, Israeli Ambassador Ephraim Evron said Israel has formally asked the United States to reconsider the proposed sale.]

The vehement protect represented Israel's shapest break yet with the Reagan administration and seemed to go against the grain of the harmony Begin and U.S. Sectrary of State Alexander M. Haig Jr. reached in talks here last month about Israel's place in the concept of a "strategic consensus" against Soviet interference in the region.

Israeli leaders had been all but estatic about the new U.S. administration's stated insistence that Israel should maintain a quantitative edge in the balance of power in the Middle East. But the arms sale decision, apart from upsetting that view, seemed to sour relations between the United States and Israel just at a time when Haig appeared to have made some headway on his first official visit abroad as secretary of state.

However, Israeli Foreign Ministry sources made it clear today that the dangers posed by Saudi Arabia's acquisition of sophisticated offensive weaponry far outweigh any potential libility to relations with the Reagan administration, or to Haig's own jockeying for U.S. foreign policy preeminence.

Israeli Foreign Ministry officials said that "friends in the United States" would be expected to take into account the prime minister's statement, and that "we will do whatever we can to stop the sale."

The statements are interpreted as a signal to the organized American Jewish lobby to wage an intensive fight in Congress against the proposed sale of the radar recommaissance planes. Reagan's decision could be blocked only by majority votes in both the House and Senate within 30 days afrer formal notification by the administration that it intends to go through with the deal.

The White House has indicated it will postpone for three or four months presentation of the formal notice, which will carry the fight beyond the June 30 general election in Israel.

When the U.S. administration first announced plans to sell Saudi Arabia fuel pods, advanced missiles and other equipment to boost the operational range of 60 F15 jets that the Saudis have already ordered, top Israel officials indicated a readiness to avoid a head-long confrontation with the United States that probably would be lost in the end anyway. In exchange, they reportedly sought an extra $600 million in military aid to Israel.

However, the inclusion in the $2.5 billion package of five airborne warning and control systems (AWACS) aircraft -- coupled with growing Senate opposition to the deal -- has encouraged many Israeli officials to back an all-out Jewish-American lobbying effort. The Israelis have argued that the AWACS -- Boeing 707s with sophisticated radar and electronic surveillance gear -- would give the Saudis the ability to collect vital intelligence data about Israeli defenses and pass it to Israel's enemies in the Arab world.

A senior aide to the prime minister refused today to be drawn into a discussion of a lobbying campaign, saying, "The question of what Congress does is not in our hands." But, he added, "we wanted to make certain that everyone in the United States understands our position on the sale."

The prime minister's statement sought to cast doubt on Saudi Arabia's reliability with such advanced weapons systems at its disposal, saying, "Saudi Arabia totally rejects the Camp David peace accords, peace with Israel and recognition of Israel.

"Saudi Arabia supplies massive financial support to terrorist organizations, and it is from that country that the call of jihad [holy war] against Israel came. The supply of sophisticated offensive weaponry to this country will undermine peace in the Middle East and create a grave danger to the security of Israel," it declared.

Dan Pattir, the prime minister's spokesman, said Begin elaborated on the statement in his meeting with Lewis, presenting additional arguments against the sale.

Foreign Ministry sources said they were unimpressed by Reagan administration arguments that the administration of former president Jimmy Carter committed the United States to the weapons sale.

"It doesn't matter," a Foreign Ministery official said. "We are opposed to the sale, whether it came from Carter or Reagan. Remember, in the beginning it was the intent of the United States to sell the planes [F15s] without the additional equipment. We see it as a kind of escalation of the sale of weaponry to Saudi Arabia."

When pressed on whether American Jewish organizations and the pro-Israel lobby, the American-Israel Public Affairs Committee and launch the kind of pressure campaign it did before the 1978 sale of the original 60 F15s, the Foreign Ministry official replied, "Your question is a tactical one. Our aim is to avoid the sale of the planes, and we will do whatever we can to stop the sale."

Deputy Defense Minister Moredechai Zippori tonight said Israel will "try to reason" with the United States, but failing that, will exert political pressure.

"Maybe there are some Arab pressures," Zippori said in an interview on Israeli radio. "We believe we can put enough . . .," and at this point he stopped and then continued saying, ". . . Let us say, first of all we can try to explain our problem and then try to put as much pressure as possible. Because for us, it is a question existence."

While continuing to use the threat of a direct Saudi strike as an arguement against the sale, Israeli leaders have widened the thrust of their case considerably, examining the arms supplies against what they consider to be the broader realities of Middle East instability and the prospects of longevity of the Saudi monarchy.

The Israelis have repeatedly warned that in the event of a revolution in Saudi Arabia, the highly secret AWACS could fall into the hands of either a radical government or the Soviet Union.

[In Cario, however, Foreign Minister Kamal Hassan Ali, welcomed the Reagan administration decision, wire services reported. In an interview, he said he was pleased with the decision because every country in the Middle East has to be strengthened "to face intervention."]