Syrian President Hafez Assad, in his first public assessment of the camp David peace agreements, said last night that Egyptian President Anwar Sadat had "defected to the enemy" by giving up "not only Jerusalem but the whole Arab cause."

Speaking softly and without passion as he opened a summit conference of Arab leaders who oppose the Sadat initiative, Assad said Sorrowfully that Sadat; Syria's ally in the 1973 war with Israel, had "left us alone in the trenches."

His remarks offered little encouragement to American officials who hoped that Secretary of State Cyrus Vance, when he comes here this weekend, might be able to cajole Assad into at least tolerating the agreements worked out by Israel and Egypt at Camp David.

As at previous meetings of the Arab hardliners, however, Assad did not back off from his professed desire to seek a negotiated peace with Israel in other forums and on other terms. Thus his split with Egypt remains more tactical than strategic, as it has for the last year.

Assad was addressing the opening of a meeting of the "Steadfastness and Confrontation Front," which groups the leaders of Syria. Libya, Algeria, South Yemen and the Palestine Liberation Organization in an anti-Sadat alliance.

The five gathered at a five-sided table in the ball-room of the Sheraton Hotel which has been cleared of paying guests and sealed off by Syrian troops in red berets. Syrian air force jets flew over the hotel during the day but these security arrangements were the only unusual signs in a capital that was otherwise calm.

Only Assad and PLO Chairman Yasser Arafat spoke at the formal opening session. Libyan leader Muammar Qaddafi, Algerian President Houari Boumedienne and All Nasser Mohammad of South Yemen declined offers from Assad to take the microphone while the press was still in the room.

Another notable but silent participant was George Habash, leader of the Popular Front for the Liberation of Palestine. Habash had not set foot in Syria - not openly, at least - since he escaped from a Damascus prison 10 years ago and earned a high spot on the Syrians' most-wanted list.

As was predictable, Arafat denounced the Camp David summit as an American imperialist conspiracy. "No to deviation from the cause, no to treason, no to surrender," he said. "Yes to challenge and steadfastness, yes to continuation of the struggle."

Although the Syrian media wasted no time in attacking the Camp Daivd accords, neither Assad nor any other government official had spoken out publicly about them until last night.

Sadat, Assad said according to an unofficial translation "forgot all his speeches and promises" in which he said he would not give up any occupied Arab land and would not make a separate peace with Israel.

He asked how it was possible for the Camp David conferees to sign agreements requiring participation of Jordan when the Jordanians had not been consulted.

"You have heard Jordan's opinion in the official statement," he said, referring to a Jordanian Cabinet declaration that Amman is not bound by agreements to which it was not a party.

King Hussein's final decision on the accords, however, was expected to depend in large measure on signals from Saudi Arabia, which helps inance his rule. Assad failed in a direct appeal to Saudi Arabia to withdraw financial support from Egypt after Sadat's trip to Jersusalem last November.

While the Saudis have criticized the Camp David accords, they also have said they would not interfere with any Arab state that sought to regain territory through negotiations. If they apply the same principle to Jordan, that could enable Hussein to withstand the presure sure to be put on him by Syria not to take part in the West Bank negotiations proposed by the Camp Daivd summit.

Assad went so far as to suggest that Sadat and Israeli Prime Minister Menanchem Begin might be preparing a joint military action against Syria "in the near future." He did not explain what led him to think that.

Although the participants in this meeting are united in their opposition to what Sadat has done, it is not clear what they expect to do about it, especially in the absence of Iraq.

Iraq refused to join the front, ostensibly because it was not militant enough, but also because it is under the aegis of Syria, Iraq's ideological rival.

[In Baghdad, the government-controlled newspapers criticized the Camp David resolutions for the second straight day, UPI reported. Al Thawra urged the Arabs to "expose the conspiracy and collaborators" and stressed that "liberation and victory over the enemies of the nation require the rejection of capitulationist settlements."]

The Damascus summit was scheduled after the Camp David meeting was announced but before its results were known. Diplomatic observers believe the participants arranged the meeting in the belief the Camp David summit would fail and that they expected to be looking for ways to bring Sadat back into a unified struggle against Israel.

This is the third meeting of the front, which was formed at a conference in Tripoli, Libya, shortly after Sadat went to Jerusalem last November. The first two failed to develop any effective counterstrategy, and Sadat, undeterred by its criticism, has meanwhile presented the hardliners with another fait accompli.

It does not appear likely that they will be any more effective in detailing Sadat this time, partly because they have little leverage over Egypt and partly because the members of the group themselves have conflicting interests.

Libya and Algeria reject the existence of Israel altogether and want no Arab dealings with it. Syria is publicly committed to seeking a negotiated peace with Israel on the basis of U.N. Security Council Resolution 242 For the Syrians, the hardliner front is essentially an anti-Sadat organization. In the context of this meeting they are obliged to take a hard line, observers here say, but their basic policy is not changed.