Israeli Prime Minister Menachem Begin said today that he is confident that Egypt will adhere to the "letter and the spirit" of the Camp David accords and will fulfill its obligations toward a comprehensive peace despite the change of government in that country.

In a 90-minute interview, his first since the assassination of Egyptian president Anwar Sadat, Begin's mood was largely one of calm, almost serene assurance that the new leadership in Cairo will not sidetrack negotiations for full peace nor disturb normalization of relations between the two former enemies.

"In letter, in spirit, to the date and to the dot on the i's, all the commitments will be carried out bilaterally," Begin said, referring to the agreements, which call for continued peace between the two countries and Israel's return of the remaining occupied portion of the Sinai Peninsula to Egypt by April 25.

Begin also rejected a suggestion made yesterday by former prime minister Yitzhak Rabin that Egypt and the United States should be asked to reaffirm the Camp David peace treaty, in light of Sadat's assassination and since neither President Reagan nor Egyptian President Hosni Mubarak were signatories.

Begin likened the Egyptian-Israeli treaty to the Constantinople convention of 1888, whose provisions on the Suez Canal, he noted, were endorsed by leaders who are no longer alive and were ratified by countries that no longer exist. Begin called the Camp David agreements "eternal."

Begin said the breach of any treaty, like the observance of one, must be bilateral, and that while "theoretically" it is possible that Egypt will violate the 1979 treaty and force Israel to follow suit, he all but rejected that possibility.

"I think the Egyptians, as intelligent people, will have to take into consideration that if they renounce the peace treaty, then Israel will act appropriately. We will not sit idly by and say, 'All right, you renounce the treaty, it doesn't exist and what happened in connection with the peace treaty will continue,' " Begin said. The reference appeared to be to the absence of an Israeli presence in the Sinai.

Begin added, "I cannot say what measures we shall undertake, but I can say there are only two possibilities: either we respect and carry out the peace treaty and its commitments bilaterally, or bilaterally they are not observed."

Even to discuss requiring any party to the treaty to reaffirm pledges already made, Begin said, could be interpreted as casting some doubt on the original document.

When asked what Israel's reaction would be if Mubarak showed an interest in a peace initiative outside the framework of Camp David, such as European Common Market peace proposals or the Saudi peace plan, Begin replied:

"I don't foresee such conditions. Theoretically, I can say, if the Egyptians should suddenly decide to renounce the treaty, or if they should commit a breach of the conditions and, for instance, send up to our borders 50,000 soldiers of their Army, then of course the whole situation is changed . . . . But I don't foresee such a possibility."

He did not elaborate on what Egyptian actions short of such a blatant breach would compel Israel to regard the treaty as annulled, other than to mention the closing of embassies and a public declaration against the accords.

Begin said Sadat and Mubarak, had convinced him that the Israeli Sinai withdrawal would not change Egypt's attitude.

He said Sadat had repeatedly told him he had kept Mubarak fully informed about the 12 Egyptian-Israeli summit meetings and that in the last one, in Alexandria in August, Sadat had been more positive than ever about normalization and the future of negotiations for Palestinian autonomy in the occupied West Bank and Gaza Strip.

Begin said that while Egypt and Israel might not be able to reach an autonomy agreement by the April deadline for withdrawal from the Sinai, there is nothing to prevent the negotiations from continuing indefinitely. He said he expected Egypt and Israel "in a very short time" to reach agreement on the election of an autonomous council for the occupied territories.

After that agreement, Israel and Egypt will continue negotiating the thornier issues of powers and responsibilities of the council, security, land and water rights and the question of East Jerusalem, which Israel annexed after capturing in the 1967 war.

But Begin said the "key to the whole situation, the real breakthrough" will be agreement on the election of the Palestinian council, which, he said, could give momentum toward solution of other issues. If the two sides cannot agree on the elections, Begin added, "we have the status quo."

Begin said he will "strongly advocate" that when ministerial-level talks resume next week in Tel Aviv, the negotiators concentrate on the elections and not get bogged down in issues that should be addressed only later.

During the wide-ranging interview, Begin:

Gave assurances that militant Jewish settlers who are preparing for a confrontation in the northern Sinai in hopes of scuttling the treaty will be evacuated by April 25. He said he, as well as the settlers, would be pained in leaving the Sinai, but added, "One has to prefer the difficulties of peace over the suffering of war."

Said he has no regrets that Israel asserted its opposition to the planned U.S. sale to Saudi Arabia of radar surveillance aircraft and enhanced F15 strike capability. "We believe our stand is absolutely right, absolutely right, without any doubt whatsoever," Begin added.

Said he was certain the U.S.-backed multinational peacekeeping force for the Sinai would be in place in time. He said he was encouraged France appeared willing to join the force.

Reiterated his rejection of the Saudi peace plan, saying it would lead to the dismantling of the Jewish state. He said the phrase "recognize Israel's right to exist" is "foolish" and "revolting" because no other nation is required to justify its right to exist.

Said he was puzzled by statements made by former U.S. presidents Gerald Ford and Jimmy Carter after the Sadat funeral, suggesting a U.S. dialogue with the Palestine Liberation Organization. "It was not a healthy statement under the prevailing international situation, coming from ex-presidents of the United States," Begin said.

Meanwhile, in remarks that were in striking contrast to Begin's mood of optimism, the Israeli Army chief-of-staff, Lt. Gen. Rafael Eitan, said today that Sadat's assassination had destroyed a "pillar" of the peace process and therefore made the future uncertain.

In an interview in the Hebrew newspaper Maariv, Eitan said, "President Sadat was a central figure in the Middle East and around him he created a process that he initiated and led. From an international point of view, he was the central pillar of the process . . . . The murder destroyed the central pillar. It is still too early to say if also the roof will go now. Maybe not. The roof is the process and the American involvement in the key Arab state, Egypt."

Eitan, who stressed, as he always does, that he was speaking from a military commander's point of view and not representing the position of the government, added, "From a military point of view, I cannot notice now any change. It is possible that under the surface there are changes which we do not notice. One has to point out that the murder came from within the Egyptian Army. Today we cannot say this act weakened the Army as a homogenous force that supports the regime, and also what is the loyalty to the regime of President Mubarak."

But, Eitan warned, the assassination could encourage outside "processes" against Egyptian stability, or operations by underground groups.