Arab leaders gathered here today for a summit that is expected to decide the Arab world's response to President Reagan's new Middle East peace initiative and a new strategy to deal with Israel following its invasion of Lebanon.

These two events have placed enormous pressure on Arab nations to agree on their own initiatives and terms for living in peace with the Jewish state.

It is already clear, however, that not all heads of state of the 22 Arab League nations will be attending this summit, which is being described in the Moroccan press as "decisive" for the Arab world.

For various reasons, it appears that nine chiefs of state will be missing, including President Saddam Hussein of Iraq, Libyan leader Muammar Qaddafi, President Chadli Bendjedid of Algeria, President Elias Sarkis of Lebanon and President Hosni Mubarak of Egypt.

By nightfall, 12 Arab leaders and four delegations headed by lower-level officials were assembled here. Among the leaders were perhaps those most important for deciding the next step in Arab strategy: Saudi Arabia's King Fahd, King Hussein of Jordan, Syrian President Hafez Assad and King Hassan of Morocco.

In addition, Palestine Liberation Organization Chairman Yasser Arafat is scheduled to arrive here Monday morning in time for the official opening of the summit. This week he established a new headquarters in Tunis after the PLO's evacuation from Beirut.

U.S. special envoy, retired general Vernon Walters, arrived in Algiers Sunday with a message from Reagan to Bendjedid, Associated Press reported.

The conference is officially a reconvening of the 12th summit of the 22-member Arab League. That meeting broke up here last November over the eight-point peace plan submitted by then-crown prince Fahd of Saudi Arabia.

The Fahd plan and another to be presented on behalf of Tunisian President Habib Bourguiba calling for the establishment of a Palestinian state alongside Israel on the basis of a 1947 U.N. resolution, are expected to form the basis of summit discussions.

Both proposals include an Arab recognition of Israel, which President Reagan has called imperative for any breakthrough in the search for a lasting settlement to the Middle East conflict.

The controversial point seven of the Fahd plan contains only an implicit recognition, stating that all states in the region have a right to live in peace, without specifically naming Israel . Even this was too much for the radical Arab leaders to accept at last November's summit. Assad, Qaddafi and Bendjedid refused to attend the summit then, although Arafat, who was involved in drafting the Fahd plan, was present.

One indication of changing thinking among the Arab world's radicals under the impact of the Israeli invasion was the early arrival of the Syrian president here.

His presence and cooperation in reconvening the summit would seem to signal Assad's acknowledgment of the need for a united Arab stance and his own need for wider Arab support in Syria's continuing military confrontation with Israel in Lebanon.

In addition, the summit was scheduled to discuss a request by the Lebanese government for an end to the mandate of the Arab peace-keeping force in Lebanon which consists entirely of Syrian troops.

The cancellation of the mandate would deprive Syria of any legal basis for retaining its forces in Lebanon's eastern Bekaa Valley, which it regards as vital to its own defense against Israel.

In any case, there were indications today that Lebanon might not be prepared to press the issue at this time. Only a low-ranking minister of state, Joseph Abu Khater, showed up today to represent Lebanon, which had asked for a postponement of the summit.

Still unknown is the attitude of the PLO toward the Fahd and Bourguiba proposals now, as well as toward Reagan's idea for Palestinian self-government in association with Jordan, but short of an independent state, in the Israeli-occupied West Bank and Gaza Strip.

PLO leaders have said they see some positive elements in the Reagan initiative but are hardly likely to give up their struggle for an independent state, which the American president opposed in his speech last week. proposals now, as well as toward Reagan's idea for Palestinian self-government in association with Jordan, but short of an independent state, in the Israeli-occupied West Bank and Gaza Strip.

PLO leaders have said they see some positive elements in the Reagan initiative but are hardly likely to give up their struggle for an independent state, which the American president opposed in his speech last week.