The Israeli government today formally and emphatically rejected President Reagan's Middle East peace proposals, asserting that, in the words of Prime Minister Menachem Begin, to accept them would amount to a "betrayal" of Israel.

In a lengthy resolution unanimously adopted at a special Cabinet meeting this morning, the Begin government declared that the Reagan suggestions "seriously deviate" from and "contradict" the Camp David peace accords, which Israel insists are the only basis on which it will negotiate the future status of the Israeli-occupied West Bank and Gaza Strip.

Since the new American proposals "could create a serious danger to Israel, its security and its future, the government of Israel has resolved that on the basis of these positions it will not enter into any negotiations with any party," a statement issued to reporters following the Cabinet meeting said.

The statement said that Israel remained ready to hold talks on Palestinian autonomy in the occupied areas "in total conformity" with the Camp David agreements.

Cabinet Secretary Dan Meridor said that government ministers had expressed "astonishment and bitterness" at the president's message and charged that the United States had failed to consult with Israel before making any Middle East peace initiative.

The Israeli opposition Labor Party took a far more sympathetic stance toward the American proposals. It issued a statement calling them a "basis for dialogue with the United States" and said that the "positive parts" of President Reagan's message should be accepted and other aspects made subject to negotiation.

Israel radio quoted Begin as saying that when he received the message from U.S. Ambassador Samuel W. Lewis he called it his "saddest day as prime minister." According to the government press office, which quoted "Israeli sources," Begin told Lewis that "any Israeli government which would accept [this] plan would be betraying its people."

The formal Israeli response and Begin's personal displeasure were conveyed directly to Defense Secretary Caspar W. Weinberger late today during a meeting with the prime minister in the coastal resort city of Nahariyya.

American officials said tonight that Begin did most of the talking during the one-hour, 20-minute meeting, providing a "frank" explanation of the Cabinet decision. "The secretary responded with a word or two, expressing hope they would not be dismissed out of hand," the official said.

Several Israeli officials complained about "lack of prior consultation" despite the fact that Reagan's letter was delivered Tuesday and then its contents were leaked to Israeli state radio shortly afterward. The reports in the Israeli media of the proposals reportedly prompted the president to deliver the nationally televised speech on his suggestions last night rather than at a later date as originally planned.

In his letter, the president called for a freeze on new and existing Jewish settlements in the occupied territories and suggested that the future status of the West Bank and Gaza should be linked to Jordan.

A second note from Reagan was delivered to Begin last night and reportedly pledged that the United States would make no moves that would "take Israel by surprise." But it apparently did not placate Begin or soften the Israeli response.

The lengthy Cabinet resolution amounted to a detailed and argumentative rebuttal of Reagan's proposals that appeared to leave virtually no room for compromise.

On the question of settlement in the West Bank, it said, "Such settlement is a Jewish inalienable right and an integral part of our national security. Therefore, there shall be no settlement freeze. We shall continue to establish them in accordance with our natural right."

The most extreme nationalist elements within the Begin government continued to call today for accelerated settlement activity or the outright annexation of the occupied territories as a concrete response to the American proposals, but apparently there has been no decision to take such drastic steps at this time.

The rejection of the proposal that the West Bank and Gaza be linked to Jordan in the future was equally blunt.

"Were the American plan to be implemented, there would be nothing to prevent [Jordan's] King Hussein from inviting his new found friend [Palestine Liberation Organization chairman] Yasser Arafat to come to Nablus [on the West Bank] and hand the rule over to him," the statement said. "Thus would come into being a Palestinian state which would conclude a pact with Soviet Russia and arm itself with every kind of modern weaponry."

The Cabinet resolution mentioned seven points that were said to be contained in the Reagan message and in each case flatly rejected them as being either not part of the Camp David accords or a misinterpretation of the 1978 agreement among Israel, Egypt and the United States.

Before his meeting with Begin, Weinberger spent most of the day with Israeli Defense Minister Ariel Sharon, inspecting Israeli weapons, manufacturing plants near Tel Aviv and military installations in the West Bank and the recently annexed Golan Heights.

This was Weinberger's first trip to Israel, and Pentagon officials said he was seeking a thorough understanding of Israeli security needs.

The officials said they understood that some of the main points of controversy in the American-Israeli military relationship -- for example, the suspension of delivery of U.S.-made cluster bombs after their use by Israel in Lebanon -- did not come up in the Weinberger-Sharon talks.

Weinberger is to meet tomorrow with Israeli Foreign Minister Yitzhak Shamir and then fly to Cairo for talks with Egyptian officials.