The Israeli parliament today strongly endorsed Israel's rejection of President Reagan's Middle East peace initiative after Prime Minister Menachem Begin taunted his opposition critics by threatening to call early national elections over the issue next spring and saying that the voters would support him.

Begin opened a defense of government policy by announcing that Israeli jets had destroyed a Soviet-made SA9 ground-to-air missile battery in Syrian-controlled eastern Lebanon. Both Begin and military officials warned that Israel would not tolerate the presence of such weapons in Lebanon. Details on Page A29.

The parliament, or Knesset, voted 50 to 36 to back a resolution supporting the Cabinet's earlier unanimous rejection of the American proposals. A few hours earlier, the Knesset also formally endorsed the government's conduct of the war in Lebanon.

The debate over Reagan's initiative was often raucous, featuring frequent shouted interruptions of Begin's concluding speech. But the prime minister made it clear that he does not fear a popular test of strength with the opposition Labor Party over Reagan's call for a freeze on new Jewish settlements in the West Bank and Gaza Strip and the eventual linking of the occupied territories with Jordan.

Addressing himself to Labor member Victor Shemtov, who contended that a majority of Israelis favor a "territorial compromise" in the West Bank and Gaza in return for peace with Jordan, Begin said:

"Let's agree that elections be held in May or June. Do you agree? If you do, we'll prove who speaks for the majority."

The remark came in the midst of a highly political speech and was not a formal call for elections before the Begin government's term expires in 1985. But it reflected Begin's political confidence in the wake of the war in Lebanon, which, according to recent public opinion polls here, has boosted the prime minister personally and his ruling Likud bloc to the highest levels of public support that they ever have enjoyed.

Begin's newly strengthened position represents a major hurdle to the Reagan administration's hope that a combination of external pressure and internal Israeli discontent with Begin's hard-line policies will force the government to retreat from its total rejection of the president's proposals.

It is widely assumed here that Begin and other Likud leaders would like to call early elections to tighten their grip on the government. Begin currently heads a coalition government that depends in part on support from a small, extremist religious party that is causing domestic problems by making demands such as for the shutting down of the Israeli national airline El Al on the Jewish sabbath and holy days.

But another, more moderate religious party in the coalition opposes early elections for fear of suffering fresh political losses. As a result, Begin might need the tacit agreement of the main opposition Labor Party in order to dissolve the Knesset and force an electoral test of strength.

In his speech, Begin noted that there have been published suggestions in the United States that the American proposals, if pressed by Washington, could lead to the downfall of the Begin government. Citing The Washington Post, The New York Times and Times columnist Anthony Lewis by name, he said, "None of these so-called democratic papers will topple governments or prime ministers. The people of Israel will democratically elect our government and prime minister."

Today's debate was spirited--one Labor member was ejected from the Knesset for repeatedly interrupting Begin with shouts--but the outcome never was in doubt. The Labor Party has endorsed the Reagan proposals as a good basis for negotiations, but it has not yet mounted a major campaign to gain public support for them.

Urging adoption of the Labor position, Knesset member Shlomo Hillel charged that the government's adamant opposition to the Reagan proposals had created the "unprecedented situation" in which the Arab leaders who were currently meeting in Morocco appeared more moderate than the Israeli leadership.

Begin said that despite the sharp Israeli rejection of the Reagan proposals, overall U.S.-Israeli relations were not at stake in the dispute.

"This plan died the minute it was born," he said. "Perhaps in the future there will be no trace of it, but American-Israeli friendship will remain."

Begin said that while the dispute was a political matter to the president, "For us it is our homeland, our country, the land of our fathers and sons. Judea and Samaria the biblical names for the West Bank belong to the Jewish people for generations to come. Nothing will change this. We shall win this battle."

Speaking before Begin, Foreign Minister Yitzhak Shamir said that "no self-respecting Israeli government" could even discuss the possibility of halting Jewish settlements in the West Bank.

Earlier today, by a vote of 50 to 40, the Knesset endorsed the government's conduct of the war in Lebanon after rejecting a Labor resolution that criticized the decision to send the invading Israeli forces deeper into Lebanon after they had completed the initially announced goal of clearing the Palestine Liberation Organization from a 25-mile zone beyond the Israeli border.

Defense Minister Ariel Sharon said that Israel would seek a "security zone" in southern Lebanon about 25-30 miles wide that would be "free of threatening artillery or other terrorist forces."

The statement went further than comments by Sharon in a speech Monday, when he said Israel would insist on such a zone if Lebanon did not sign a peace treaty with Israel. Sharon did not say whether Israeli soldiers necessarily would remain in the zone.

Sharon said that the Western press had grossly exaggerated the damage caused by Israeli shelling during the siege of Beirut, asserting that only 40 to 50 buildings had been damaged. Noting that U.S. special envoy Philip C. Habib had been awarded the Medal of Freedom for mediating in the PLO withdrawal talks, Sharon said that the Israeli people deserved "the monument of freedom" for their role in the war.