Surprise diplomatic initiatives last week by Israel and Jordan sparked a renewed sense here of movement toward a Middle East peace, eclipsing for many Lebanese the despair that had been felt after the bombing of the U.S. Embassy annex in east Beirut.

What dominated Beirut headlines as investigators sifted through the embassy rubble was not mourning for the American and Lebanese dead or speculation about the perpetrators of the attack, but the prospect of a political realignment in the Arab world and a possible end in sight for the long and bitter Israeli occupation of southern Lebanon.

The Sept. 20 bombing played no apparent part in the unexpected decisions by either Israeli Prime Minister Shimon Peres to accelerate efforts toward a withdrawal of forces from southern Lebanon or by King Hussein of Jordan to reestablish diplomatic relations with Egypt. Both moves have prompted measured optimism here, however -- or at least provided a new set of variables with which to speculate -- partially assuaging the feelings of hopelessness that followed the terrorist strike against the United States.

Reflecting this swing was the visit by U.S. Assistant Secretary of State Richard W. Murphy. Arriving from Washington one day after the blast in Aukar, Murphy seemed at first to be here in connection with an official investigation into the attack.

But his trip was soon extended into a series of high-level contacts that took the U.S. envoy to Jerusalem, Amman and Cairo, twice to Damascus and back to Beirut. He returned to Washington yesterday. What had appeared to begin as a mission of mourning assumed the character of a mission of mediation, reviving several policy options for the Reagan administration in the Middle East, in the view of several analysts here.

The first option is to help facilitate an Israeli withdrawal from southern Lebanon. Another involves support for the reconciliation between Jordan and Egypt and for possible further moves by King Hussein toward agreement with Palestinian leader Yasser Arafat, leading in time to peace talks with Israel.

U.S. involvement in either problem area could upset an apparent new understanding with Syria toward which Washington seems to be edging. Damascus is still a power to be reckoned with in the region. Significantly, the Reagan administration so far has avoided insinuations of Syrian involvement in the embassy bombing. In the past, U.S. intelligence sources have linked Syria and Iran to the shadowy terrorist group code-named "Islamic Jihad," which claimed responsibility for all three attacks on U.S. facilities in Beirut in the past 17 months.

Syria's bid to become a kind of regional spokesman for the Arab cause was undercut by Jordan's alignment with Egypt, which promises to provide a moderate counterweight to the Syrian and Libyan hard-line position. But Syrian President Hafez Assad still is central to settling the Lebanese crisis as well as to getting Israel out of its increasingly costly 27-month occupation of southern Lebanon.

Syria rejected today a call issued last night by Morocco's King Hassan for a special Arab summit to discuss Jordan's decision to reestablish ties with Egypt, Reuter reported from Damascus. In Amman, Jordanian Deputy Prime Minister Suleiman Arar defended the Jordanian action in a radio interview. "Only one or two Arab countries have attacked our decision and threatened us," he said. "They can go jump into the sea."

Recognizing Assad's key role, Israel dropped last week its longstanding insistence that a Syrian withdrawal from eastern and northern Lebanon be simultaneous with any Israeli pullout. Together, the two armies hold about two-thirds of Lebanon.

In turn, the Israelis are seeking certain assurances from Assad of help in keeping terrorists from reestablishing footholds in southern Lebanon for raids against Israeli settlements.

Another potential stumbling block that emerged in public statements this weekend are differences over who should replace the Israelis in the south. Moslem Lebanese officials said they favored expanding the size and mandate of the small United Nations force currently in the south, in addition to deploying reconstructed Lebanese Army units. But Israeli Cabinet secretary Yossi Beilin was quoted today as advocating a policing role also for the 2,100-strong "South Lebanese Army" a Christian-led, Israeli-trained force resented by Lebanese Moslems.

Meanwhile, the flurry of new diplomatic initiatives may dampen for the moment efforts to resolve Lebanon's internal crisis. All sides are likely to want to wait and see whether the international moves minimize the concessions demanded of them. As the leftist Beirut daily As Safir wrote: "All the dossiers are placed in the freezer until after the movements in the region" have played themselves out.

So far even pressure by Syria on Lebanon's rival factions to pursue talks on political reform has failed to produce agreement. Cabinet conclaves adjourned last week with no consensus on such central issues as an increase in parliament's membership from 98 to 120 to make the legislative chamber half Moslem, half Christian, or the appointment of more Moslems and Druzes to senior positions in the Christian-dominated civil service.