Israeli Prime Minister Menachem Begin yesterday defended his country's invasion of Lebanon in a two-hour, private meeting with a group of senators that some participants later described as filled with angry questioning and criticism of Israel.

"In my eight years in Washington, I have never seen such an angry session with a foreign head of state," was the comment of Sen. Paul Tsongas (D-Mass.), who normally is among the most stalwart of Israel's supporters in Congress.

His assessment, which was echoed by several of the approximately 36 senators who attended, indicated that, despite the soft line taken by President Reagan following his meeting with Begin on Monday, there is considerable misgiving and uneasiness in this country about the heavy casualties and destruction resulting from the Israeli incursion.

Whether the concern will translate into an erosion of Israel's support in Congress and lead to efforts to use U.S. military and economic support as a lever to curb Israeli conduct was not immediately clear. Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) noted: "I think anytime you have a conflict of this magnitude it puts a strain on friendships, but I don't think there will be any permament dislocation . . . ."

Begin, who arrived in the United States last week warning that Israel will not submit to any "friendly pressure" before its objectives in Lebanon are accomplished, reportedly maintained this tough attitude during yesterday's meeting by responding in a manner that some called "confrontational."

Said Sen. Larry Pressler (R-S.D.): "This is the first time I have seen such a confrontation between the prime minister of Israel and senators in terms of head-to-head disagreement. He is taking question after question and just hitting them head on. He is not budging an inch."

In talking with reporters afterward, Begin acknowledged that it had been a "lively discussion" and added: "I enjoyed the session very much. I believe in liberty, that free men should freely discuss problems and if they have differences of opinion, they should voice them in sincerity . . . . I believe the alliance is deeper than it used to be."

His attitude appeared to reinforce the opinion of Israeli sources that Begin believes he has accomplished the main purpose of his visit. That was to convince Reagan not to follow the advice of senior officials such as Defense Secretary Caspar W. Weinberger, who wanted a U.S. rebuke of Israel, and instead endorse Israel's demands for enhanced security through withdrawal of Syrian troops from Lebanon and creation of a demilitarized zone, cleared of Palestinian guerrillas, in southern Lebanon.

According to accounts of the Reagan-Begin meeting, the prime minister won the president's backing by giving him a forceful account of the danger the Palestinians posed to residents of northern Israel and by reinforcing the argument that Israel's actions can be turned to the advantage of U.S. policy by setting the stage for a new stability within Lebanon and a possible new start on resolving the Palestinian problem through negotiation.

That idea has the backing of Secretary of State Alexander M. Haig Jr., who is understood to have advocated a soft U.S. response in the hope that the so-called "new reality" within Lebanon can be used to help the central government reassert its control over the country's many feuding factions. Haig also is understood to believe that supporting Begin on Lebanon will obligate him to make concessions later on autonomy for Palestinians in Israeli-occupied territories.

Begin is reported to believe that, having accomplished his main goal of winning Reagan over, those pockets of opposition to Israel's action that remain in Congress and U.S. public opinion can be overcome through the efforts of the American-Jewish community, which has shown no signs of wavering in its support of Israel. He also thinks the opposition will dissipate if events do lead to long-range stability in Lebanon.

In private, however, administration sources confirmed a report circulated by a Washington newsletter, the Middle East Policy Survey, that Reagan's special mediator in Lebanon, Philip C. Habib, is "furious" at both Haig and Begin for allegedly misleading him about developments and policy shifts that he believes have harmed his efforts. The sources denied, though, that Habib has threatened to resign.

According to those present at the Senate meeting, much of the discussion involved questions of whether Israel had used U.S.-supplied cluster bombs, capable of causing heavy civilian casualties, in violation of American law and assurances given by Israel in 1978 that it would not do so. Tsongas said Begin had replied to his questions about the use of such weapons by saying he did not know; and at the State Department, Fischer said the Israeli government still has not replied to official U.S. requests for information.

Weinberger, who on Sunday publicly criticized Israel in harsh terms, continued yesterday to make clear his opposition to the invasion. In an interview with the Mutual Broadcasting System, he charged that U.S. participation in a peace-keeping force for the proposed buffer zone would cost up to $2 billion a year, involve American troops in situations where they might have to kill Israelis or Arabs, and cause "immense diplomatic and political problems" for U.S. relations with the Arab world.