Gratified by President Carter's declaration of support, Egypt today continued to hold out hope that some other Arab nation might yet participate in the preliminary talks to be held here this month with Israel.

The acting foreign minister, Boutros Ghali, told reporters that Egypt would keep open invitations to the conference until about Dec. 10 in the hope that Syria, the Palestine Liberation Organization or Lebanon might change its mind and decide to attend.

Lebanon announced today that it would turn down Egypt's invitation, joining Syria and PLO, which earlier had rejected Cairo's bid. King Hussein of Jordan also reiterated that his country would only attend the Cairo talks of "all the parties concerned on the Arab side" agree to attend.

What really matters to Egypt is the response from Syria, and high officials continued to hope that despite Syria's vehememt public ciritcism of the Sadat initiative, some formula for Syrian participation might still be worked out.

Failing that, they hoped that Syria would at least stop short of all-out effort to sabotage Egypt's peace campaign, and thus leave room for later participation at a Geneva conference under terms worked out by Cairo and Israel.

"Sure there is still some hope for Syria but later perhaps," a senior Foreign Ministry official said. "It's all part of the process."

President Anwar Sadat has said publicly that he appreciates that the situation is "difficult for President [Hafez] Assad and delicate for the Syrians."

The Egyptian hope is that Syria, after polishing its credentials as an Arab hard-liner at the Tripoli conference of rejectionist Arab states, will grudgingly follow Egypt's lead and work toward the peace Assad says he wants.

Carter's press conference yesterday, in which he made the first unequivocal declaration of American support for Sadat's campaign, was televised live here and drew front'page, approving coverage in the Cairo press this morning.

It was a welcome tonic for peace-happy Egyptians who were nervous about what many people perceived as a cool U.S. response to Sadat's call for a preparatory conference here to get ready for Geneva.

It came as a particularly timely boost for Sadat because it deflected attention from the decision of Egyptian Ambassador to Yugoslavia, Morad Ghaleb, to resign today in disagreement with Sadat's policies.

Ghaleb, a former Egyptian foreign minister who served during the 1960s as Cairo's ambasador to the Soviet Union, delcared that he felt he could no longer "defend or justify the present policy line of President Sadat."

Though officials did not say so publicly, Carter's suggestion that Dec. 13 would be the starting date of the Cairo meeting apparently came as a complete surprise to Egypt, which was preparing to open the talks as early as Sunday. Ghali said again today that no date has been set.

Carter's endorsement, if not his timeable, was "what we expected in our group here," an authoritative official of the Foreign Ministry said. "The people were anxious because they thought peace would come tomorrow," he added, but Sadat's initiative "shocked everybody and the U.S. is no exception. It took some time for them to adjust."

He said Egypt was never firm on a date for the conference and the invitations said only that participants would be welcome "as of a certain date. "The Egyptians understood that "it needed time for development, time to give the U.S. a new dynamic."

As for the proposal by U.N. Secretary-General Kurt Waldheim to hold another pre-Geneva conference at the United Nations, Boutros Ghali said Egypt is for peace so "if there is another meeting talking about peace, then we have no objection to it."