Egyptian President Hosni Mubarak, in an apparent effort to soften his isolation in the Arab world, today joined a litany of Arab leaders' condemnations of Israeli occupation of Lebanon and the territories seized by Israel in the 1967 war.

Mubarak, in a speech to the seventh summit conference of nations avowing nonalignment, said the "core and crux" of the Middle East conflict is the Palestinian problem.

He called for the establishment of a Palestinian "national entity" in the Israeli-occupied West Bank and Gaza Strip by the "brotherly people of Palestine," and said that all Arabs are "in one trench facing one and the same problem . . ."

While Mubarak's comments paled by comparison to the rhetorical pitch reached by some other Arab leaders at the summit, it appeared to signal a concerted effort by the Egyptian president to heal the wounds of the 1979 Egyptian-Israeli peace treaty that stirred an anti-Egyptian furor at the nonaligned summit in Havana that year.

Coupled with a flurry of behind-the-scenes meetings here among the Arab principals of the Middle East conflict, Mubarak's remarks, while breaking no new ground for Egypt, gave rise to speculation about a new phase of detente betwen Egypt and the Arab states that have steadfastly rejected the Camp David peace accords.

Some Arab delegates, including Libya's prime minister, Abdul Salaam Jalloud, who had sought Egypt's expulsion from the Nonaligned Movement, unobtrusively left the conference hall during Mubarak's speech.

But other Arab delegates seemed to warm to the Egyptian leader's conciliatory tone, and particularly his condemnation of the presence of Israeli troops in Lebanon, which Mubarak termed "a grave precedent of intervention in the affairs of small countries."

Mubarak said a solution to the Palestinian problem should be built around "a consensus that was recently reached and aimed at replacing the Israeli occupation" with a Palestinian authority in confederation with Jordan. The consensus to which he referred was at an Arab summit last year in Fez, Morocco.

The nonaligned conference, while grappling abstractly with such issues as global nonviolence and a new international economic order, has become a venue for intense backstage discussions between Mubarak and other Arab leaders. The Egyptian president has met privately with Jordan's King Hussein for two hours, and has had contact with Palestine Liberation Organization Chairman Yasser Arafat and Lebanon's president, Amin Gemayel. In addition, Gemayel has held meetings with Hussein, Arafat and Syrian President Hafez Assad.

Arab diplomatic sources said it is highly unlikely that any formal agreements for easing the tensions between Egypt and the rest of the Arab world would be reached at the summit, but they acknowledged that the meetings reflected a progressive easing of tensions with Egypt.

Abdel Moushen Abu Maizer, spokesman of the PLO's executive committee, signaled a change of attitude when he responded today to Mubarak's reprimand of the PLO earlier this week for adopting a resolution at the Palestine National Council meeting in Algiers calling for the PLO to establish contacts with Egypt's nationalist forces, a term viewed in Cairo as meaning Egyptian political organizations opposed to the peace treaty with Israel.

Maizer stressed that the PLO had no quarrel with Egypt, and that the Algiers resolution had been "misunderstood" by the Egyptians.

But when asked about the Mubarak-Hussein meeting yesterday at which the Jordanian monarch reportedly was asked to form a delegation with the Palestinians to explore the Reagan peace plan, the PLO official said, "Any meeting between Arab leaders, if it is hostile to our cause, of course we have to oppose."

Maizer called the Reagan initiative a "call to war," saying that it ignores the rights of Palestinians. "How can we say after ignoring those facts that the United States is really sincere about peace in the Middle East," he added. That, Maizer said, is why the Algiers meeting did not consider the U.S. proposals seriously.

A parade of other Arab leaders condemned what they termed Israeli aggression in the Middle East, although the tone of some speeches was more low-key than those of previous nonaligned summits.

More intense debate was reserved by some delegates for the Soviet occupation of Afghanistan and what they termed the denial of nonaligned membership to exile Cambodian leader Prince Norodom Sihanouk.