Israel's major opposition party came out in support of President Reagan's peace proposals today and announced that it will demand a special session of Israel's parliament, the Knesset, to discuss them.

"We are going to put our case before the people," Labor Party leader Shimon Peres said.

Defense Secretary Caspar W. Weinberger, winding up a two-day visit to Israel, reaffirmed Reagan's determination to press ahead with his proposals despite their outright rejection by the Israeli government.

Reagan's suggestions represent a "carefully thought out, deeply felt position," Weinberger told reporters as he emerged from a one-hour meeting with Israeli Foreign Minister Yitzhak Shamir. "I know the president does not adopt positions lightly."

Weinberger, however, took the same low-key stance toward the intense Israeli criticism of the Reagan proposals as have other administration officials in Washington. He called his talks with Israeli officials "extremely constructive" and said there remain "a number of possibilities for consideration of the plan."

The defense secretary flew later today to Cairo for meetings with Egyptian officials before his return to Washington. While in Israel, he served as the lightning rod for Israeli anger over the president's message to Prime Minister Menachem Begin while undoubtedly gaining a clearer assessment of how much, if any, Begin is willing to retreat from his blunt and total rejection of the American suggestions this week.

Defense Minister Ariel Sharon Friday called Reagan's peace plan a bid to deprive Israel of the fruits of victory over Palestinian guerrillas and a "big mistake," United Press International reported.

The government-run Israeli radio reported that Sharon had told Weinberger, "Israel has the right and duty to reap all the benefit from its achievement, which came at the heavy cost of the lives of its sons," UPI said.

Reagan's message to Begin was delivered Tuesday night. In it, the president reiterated his support for the Camp David peace accords, but he called for a total freeze on new and existing Jewish settlements in the Israeli-occupied West Bank and Gaza Strip and suggested that the future status of the occupied territories should be linked to Jordan.

These and other points raised in the president's message were rejected out of hand yesterday by the Israeli Cabinet, which said the proposals "seriously deviate" from and "contradict" the Camp David accords.

At a news conference in Tel Aviv today, Peres said the Labor Party unanimously welcomed the Reagan proposals as "a basis for serious dialogue." The proposals, he said, could open the door to Jordanian participation in the peace process. The Labor Party leader said he has information that King Hussein is ready to make such a move.

"To the best of my knowledge," Peres said, "Jordan has agreed to come in and participate in negotiations on the basis of the American proposals. This is a major development."

The Labor Party leader also said he would be willing to campaign against the Begin government on the basis of his support for the American proposals. There was speculation here today that Begin may attempt to call early elections next year and to make continued Israeli control of the West Bank and Gaza the central issue of such an election test of strength.

The Labor Party, like the Begin government, strongly opposes the creation of an independent Palestinian state in the West Bank, but it has long advocated some form of "territorial compromise" in which Israel would withdraw from some of the territory it captured from Jordan in 1967. This is often referred to as the "Jordanian option," in which territory inhabited by West Bank Palestinians would be federated or linked to Jordan along the lines Reagan suggested.

When it was in power, the Labor Party established settlements in the West Bank for "security" purposes. But its settlement policy was not as aggressive as that of the Begin government, which views the West Bank as a part of historic Israel that is never to be surrendered.

It is not clear how much difference Peres' support for the American position will make. Israel's crushing defeat of the Palestine Liberation Organization in Lebanon appears at least temporarily to have substantially boosted Begin's political fortunes at the expense of the opposition. The Labor Party -- which supported the invasion of Lebanon but not the Israeli drive to the gates of Beirut--has been attempting to launch a national debate on the conduct of the war. But that effort has now been overtaken by the controversy over the Reagan proposals.

Weinberger, who met yesterday with Begin after the Cabinet decision, characterized the prime minister's comments as a "first sharp reaction" to the Reagan proposals. Begin and other Israeli officials have complained bitterly that the administration failed to consult with them in advance but did have discussions with Arab nations before handing Israel "a complete, prepared paper on which we are asked to agree."

The officials charged that in doing so the Reagan administration violated American commitments made by former secretary of state Henry A. Kissinger and former president Ford "not to bring plans, proposals, ideas for a Middle East peace settlement to Israel without prior consultation."

Weinberger today defended the manner in which the Reagan proposals were developed and transmitted to Israel. The administration, he said, considered it "vital to ensure that the countries that did not participate in the Camp David accords participate in this. The president's timing and procedures that were adopted were designed to try to ensure that."

It was clear from Peres' comments and the results of a public opinion poll that Weinberger indirectly referred to that the government's hard-line insistence on maintaining Israeli control over all the occupied territories is by no means a general consensus within Israel.

The poll, published in today's Jerusalem Post, showed that since the war in Lebanon the number of Israelis willing to give up parts of the West Bank has risen, from 30 percent in May to 40 percent in August.

While maintaining their adamant public opposition to Reagan's suggestions, Israeli officials today sought to tone down the extravagant rhetoric that accompanied yesterday's Cabinet declaration. Weinberger's meeting with Foreign Minister Shamir was described as "very friendly." Shamir said, "There are deep differences of view about this plan Israel cannot accept." But he added that there is no "crisis" in U.S.-Israeli relations.

In another development, Israeli radio reported that Begin met secretly in northern Israel Wednesday night with Lebanon's President-elect, Bashir Gemayel. According to the report, Begin chastised Gemayel for his recent public statements regarding Israel, and Gemayel responded by promising to seek peace with Israel once he is firmly in office.

Gemayel's office in Beirut denied that such a meeting had taken place.