Prime Minister Menachem Begin indicated yesterday that Israeli participation in a foreign ministers' conference with the United States and Egypt, as called for the President Carter, may not be as automatic as Washington had believed.

Only minutes after Vice President Walter Mondale had said he believed Israel was prepared to attend such a meeting, Begin cautioned that Israeli participation hinged on a new set of much-publicized, but as yet undelivered Egyptian peace proposals.

In what was widely viewed as an effort to put new pressure on Israel, Carter warned in remarks released Saturday that Middle East peace talks could be returned to Geneva - where the Soviet Union would have a role as co-chairman of a reactivated conference - should current U.S. efforts for a compromise between Egypt and Israel fail.

It was in this context that Carter called for a foreign ministers' meeting at which the United States might present "compromise" proposals.

Israeli officials initially appeared to be angered by the Carter statement, and U.S. officials worked throughout yesterday to attempt to soften the impact.

Mondale winds up a three-day visit to Israel today and goes to Alexandria to meet briefly with Egyptian President Anwar Sadat.

Begin's reservation about committing Israel to the London meeting before receiving the Egyptian proposals came as something of a surprise to U.S. officials, who had taken for granted that Israeli Foreign Minister Moshe Dayan would be in London for the proposed meeting.

Mondale, who met with Begin for 1 1/2 hours yesterday, indicated that he thought the Israeli government had already committed itself to attending the foreign ministers' conference.

"I think it is fair to say that the Israelis have agreed to attend the London conference. We have not yet received final approval from the Egyptians," Mondale said.

Begin however, offered a different chronology saying: "First we should get the proposals by Egypt as promised and we want to read it and study it and analyze it, and then we shall decide about a meeting which shall of course take place after we have had time to study. The should, and I hope will be, the sequence."[LINE ILLEGIBLE] As Begin and Mondale spoke with reporters in front of the prime minister's office, about 200 demonstrators from the ultra-nationalist Gush Emnuim movement waved placards declaring "The U.S. has no mandate in Israel" and began chanting so loudly that they nearly drowned out the remarks of the two leaders.

An informed Carter administration official traveling with Mondale later said that he believed Begin is seeking to keep Israel's options open in case Egypt submits an unacceptable proposal. The official stressed, however, "our expectation is that the meeting will take place."

U.S. officials said they expect to receive the Egyptian plan "sometime this week" but did not know if it will be presented to Mondale in Alexandria today. Begin said he expects the Egyptian plan to be submitted today or tomorrow.

Begin also disclosed that a date for a meeting between Israeli Defense Minister Ezer Weizman and Egyptian War Minister Mohammed Abdul Gamassi may be set as early as this week or next. Weizman has been communicating with Gamassi regularly through Israel's military negotiating committee, which is still Egypt although it has been inactive for several months.

Mondale, in what his aides termed a major foreign policy speech, last night reassured Israel that "no other interest, no other cause" will compromise the special relationship between the United States and Israel.

He warned, however, that time is of the essence in the Middle East peace process, and that if "all sides advance maximum positions" and if peace with the Arabs slips Israel's grasp, then "who can accept the terrible price of failure."

In the speech, which aides said said had been read and approved by President Carter, Mondale stressed the importance of negotiating peace on the basis of U.N. Security Council Resolution 242, which, he said, forms a "common touchstone" for negotiations. The resolution calls for Israeli withdrawal from occupied territories.

"If there is to be peace, the implicit bargain of U.N. Resolution 242 must be fulfilled," Mondale said, at the state dinner held in the Knesset.

He drew warm applause when he assailed terrorism, and specifically condemned the Palestine Liberation Organization by name.

"We condemn PLO terrorism totally and absolutely. We condemn those responsible for acts of terror and those who claim credit for them," Mondale said.

The United States has come under criticism by Israelis because of its previous refusal to characterize the PLO as a terrorist organization.

In his joint press conference with Begin, Mondale also attempted to put as good a face as possible on Carter's controversial remarks, made Friday to a group of U.S. newspaper executives, and released Saturday. Carter suggested that Israel may find Egypt's peace proposal "inadequate" and that if mediation failed to produce an Israeli-Egyptian meeting, then the stalemated peace process might have to be put either to the United Nations or a reconvened Geneva peace conference.

The vice president said Carter's remarks had been "misinterpreted by some" and that all the president did was to "observe something that is well known and recognized in international law" - that if all other efforts fail the parties "Try every way we can."

"In other words, he (Carter) was not trying to raise this as a current pospect," Mondale said.

"This problem did not come up . . . We read the statement by Presient Carter and we understood it," he said.

Despite Begin's and Mondale's disclaimers, high-ranking Israeli government officials yesterday continued to say they were perplexed and disturbed by the timing and the president's suggestion that the Geneva conference be reconvened.

Former foreign minister Abba Eban said last night: "I think it's about the worse thing to do because it means we involve the Soviet Union directly. Israel, Egypt and the United States all have an interest in direct negotiations without any Soviet involvement."

Opposition Labor Party leader Shimon Peres said: "I think it's untimely. An air of freedom without other excess hints which will create resentment is very necessary."

A top-ranking Foreign Ministry official close to Dayan called the Carter statement a "bombshell," and noted that Begin had made a conscious decision to omit from any of his public speeches during Mondale's visit any mention of Israeli grievances against the United States, such as the recent arms sales to Egypt and Saudi Arabia.

Earlier in the day, Mondale, his wife, Joan, and daughter, Eleanor, traveled by the helicopter to the Negve Desert where they laid a wreath at the tomb of the late prime minister David Ben-Gurion and dedicated a new Hubert II. Humphrey center for social research at Ben-Gurion University in Beersheba.