The war in Lebanon was "not just a misadventure, but a tragedy" for Israel and Lebanon that set back the Middle East peace process and turned into "a damaging episode for the United States," U.S. Ambassador Samuel W. Lewis said today.

In a rare on-the-record interview with U.S. journalists based in Israel, Lewis reflected on his eight years as the American ambassador here and on the event that came to dominate his tour of duty.

He said the 1982 Israeli invasion of Lebanon "was a failure largely because of basic misconceptions from the start, and that failure was assured by a lot of mistakes along the way by all of the parties, including ourselves."

He added: "The Lebanon war, the preoccupation with the Lebanon era, was a great diversion and made it impossible for Israel, the United States or Jordan to seek a way to continue the peace process. I think we would have been able to renew the peace process in 1981 or 1982 had Lebanon not diverted diplomatic and psychological energies."

The U.S. Embassy scheduled the group interview last week in connection with Lewis' pending departure as ambassador and retirement from the Foreign Service. But, like the last three years of Lewis' unusually long tenure as the U.S. ambassador here, his final days in the job have been dominated by the Lebanon war and a public dispute with the chief architect of that conflict, former defense minister Ariel Sharon.

Lewis reignited Israel's national debate on the origins of the war Wednesday in an interview broadcast on Israeli television. He said that six months before the invasion Sharon had outlined to Philip C. Habib, President Reagan's special envoy in the Middle East, and other U.S. officials "a hypothetical concept for a military operation in Lebanon" that closely resembled the plan the Israelis followed in the invasion.

Sharon vehemently denied this, accusing Lewis of "a gross lie" and charging that the ambassador was primarily responsible for "the Americans' failures in Lebanon."

Sipping a soft drink in the library of the U.S. Embassy here, Lewis opened today's interview by reading from a prepared statement reiterating his account of the Sharon-Habib meeting and saying his version was supported by documents in the embassy. He said he was making the statement because of "the need for historical accuracy."

It was not surprising that Lewis' last days here as the U.S. ambassador should be dominated by a dispute with Sharon. The deep personal animosity between the two men is widely known in Israel's political and diplomatic communities and reached a peak during the Lebanon war.

A 54-year-old native of Houston and career Foreign Service officer, Lewis will officially leave his post here next Friday and return to Washington, where he is to be a diplomat in residence at the Johns Hopkins University School of Advanced International Studies.

The White House has nominated Thomas Pickering, currently the U.S. ambassador in El Salvador, to succeed him.

In his eight years here -- a term that made him the longest-serving ambassador at any diplomatic mission in Israel -- Lewis became an extraordinarily popular figure among Israelis. He clearly enjoyed the country's casual life style, invariably appearing, as he did today, in an open-collar shirt.

For the last several weeks, Lewis and his equally popular wife, Sallie, have been the subjects of numerous honors and farewell celebrations. A new section of Israel's National Forest has been named in their honor. But there have been no tributes from the Palestinians who live in the Israeli-occupied West Bank and Gaza Strip. The Palestinians view Lewis as an unabashedly pro-Israeli diplomat, sometimes suggesting that he "should become a citizen of Israel ."

Because of the intricacies of Middle East diplomacy, Lewis has had little official contact with the Palestinians who live under Israeli occupation. U.S. dealings with the West Bank are handled through the independent American Consulate in Jerusalem.

In the interview, Lewis, who was relaxed but chose his words carefully, said on other topics:

* Israel has made some progress in attempting to reverse its economic decline but "they have a lot more to do." He said the Reagan administration has urged the Israelis to address "certain fundamental problems in the economy," but so far little has been done in this direction.

* There is "some very modest hope" that direct peace negotiations between Israel and Jordan could begin in a few months. While the Israeli occupation of the West Bank and Gaza Strip is not "irrevocable," Lewis said, "time is beginning to run" on the possibility of devising a diplomatic solution to the future of the territories.

* Former prime minister Menachem Begin violated "the spirit" of the Camp David peace accords by aggressively pursuing his policy of building new Jewish settlements in the West Bank after agreeing to the Camp David formula calling for "autonomy" for the Palestinian residents of the territory. Lewis said there was a "legitimate expectation" that settlement activity would be restrained after Camp David, and that Begin's failure to do this contributed to the collapse of the autonomy talks.

* The PLO was weakened by its military defeat in Lebanon "but by no means is a negligible factor" in Middle East diplomacy. Lewis said he continues to endorse the U.S. policy of not dealing with the PLO until it first recognizes Israel's right to exist.

Lewis declined to answer some questions. Asked if Sharon had ever indicated to him or other U.S. officials that he expected a massacre to result from the decision to send Lebanese Christian Phalangist militiamen into the Sabra and Shatila refugee camps of West Beirut in 1982, Lewis noted that U.S. officials were prevented from addressing such questions both in the Israeli investigation of the massacre and in Sharon's libel trial against Time magazine because it would set "a bad precedent."

He also declined to give his impressions of former president Jimmy Carter, who appointed him ambassador to Israel in 1977, and Reagan, who reappointed him, laughing off the question by saying he had to "save something" for a book he intends to write about the Middle East peace process.

Lewis said his advice to his successor, Pickering, will be to "get a hobby" to divert his attention from the pressures of the Middle East, and to spend as much time as possible learning about the complexities of Israeli society outside the official political-diplomatic community.

Looking back on his tour as ambassador, Lewis said the high point was clearly the 1977 visit to Israel of the late Egyptian president Anwar Sadat, a dramatic breakthrough in the violent history of the Israeli-Arab conflict. He said the low point occurred in the autumn and winter of 1982, when disagreement over the war in Lebanon and its aftermath soured the U.S.-Israeli relationship.

Lewis said that relationship is now "in the best shape as perhaps it has ever been," and added:

"We are really irrevocably bound to each other and entangled in a whole variety of ways, and it is in the nature almost of a family relationship."