The 30-year search for an Arab-Israeli peace agreement shifted to the mountaintop retreat of Camp David yesterday as President Carter welcomed Egypt's Anwar Sadat and Israel's Menachem Begin for a week or more of secluded conversations that Carter hopes to transform into serious negotiations.

Arriving separately at Andrews Air Force Base before flying by helicopter to Camp David, the Egyptian president and the Israeli prime minister each expressed determination to work for peace at the talks in brief arrival statements.

But they again expressed sharply differing assessments of goals of the summit conference and of the role Carter is to play in the trilateral talks, which begin today.

"This is no time for maneuvers and worn-out ideas," Sadat said, standing on the tarmac bathed in bright sunshine. "It is a time for magnanimity and reason." Terming the summit "a crucial crossroad" for worle peace, he said that if Carter joined him as "a full partner" in the negotiations, "together we shall overcome."

Begin arrived two hours later to a nearly identical welcome from Vice President Mondale and Secretary of State Cyrus R. Vance. He described the conferences goals in far more modest terms, saying he had come "to reach an agreement so that the peace process can continue and ultimately be crowned with peace treaties."

Vance accompanied the two leaders on their 30-minute helicopter journeys from Andrews to the presidential retreat in Maryland's Catoctin Mountains. Each embraced Carter warmly after stepping from the helicopter onto the asphalt landing pad nestled in the heavily guarded estate's neatly trimmed lawns.

Carter kept away from a small group of reporters let onto the grounds to observe the arrivals, and spent about 15 minutes with each of the two leaders explaining the lodges where they and their delegations will stay. They all then disappeared from public view for the next few days, if Carter's plans for the conference hold up.

He has imposed a news blackout on the summit's daily proceedings and has said he will stay at Camp David as long as necessary. He has turned over to Mondale day-to-day responsibility for the U.S. government while he attempts to revive the Israeli-Egyptian direct negotiations that Sadat launched with his trip to Jerusalem last November.

Sadat and Begin met again in Egypt in January, but failed to reach agreement on a set of principles that Sadat said was essential for negotiations. Talks at lower levels continued fitfully until July 30, when Sadat declared all contacts with Israel ended and triggered Carter's invitation to the two men to meet him at Camp David.

Before leaving Washington Monday, Carter said that compromises would be "mandatory" if there was to be progress. But the Egyptian and Israeli leaders continued yesterday to give no public indication that they are ready to change their deadlocked positions, especially on the central issue of Arab demands that Israel withdraw completely from the West Bank territory of the Jordan River and Begin's refusal even to consider that demand.

The three leaders have worked on their proposals for the summit in total isolation from each other and there is no agreed agenda of items to be discussed, as is usually the case in presidential summits. Carter plans to spend the first day or two of the conference in relaxed conversation with the two men before moving into serious negotiations.

He opened the conference's work by meeting privately last night with Begin, who arrived in New York on Sunday and rested at a hotel there Monday. The president will hold a similar meeting today at 10 a.m. with Sadat, who flew to Andrews from Paris in an Egyptair Boeing 707 airlines.

Dressed in an expensively tailored dark suit and managing to look both solemn and confident, the Egyptian leader received the traditional 21-gun salute and a red-carpet welcome.

After strolling across the tarmac to greet a crowd of about 200 supporters who turned out to cheer him, Sadat read a three-minute statement in which he said that, "We cannot afford to fail. No one has the right to block the road to peace."

He praised the United States as the nation "most qualified to be a full partner in the peace process," and implied that Carter has promised to fulfill that role-which the Arabs define as one of pressuring Israel-by issuing the invitations to the summit.

Sadat, who has said that he opposes a new round of negotiations unless there is substantial progress at Camp David, then boarded the Marine helicopter that normally ferries Carter between Washington and Camp David, Sadat bussed Carter on both cheeks and shook hands with him before kissing Mrs. Carter on the check.

Begin, whose arrival statement at Andrews was about half the length of Sadat's, arrived in the same helicopter about two hours later. The Israeli prime minister emerged with a broad smile on his face and walked with a firm stride toward the Carters, whom he embraced in a replica of the Sadat greeting.

Carter also shook hands with Israeli Foreign Minister Moshe Dayan, Defense Minister Ezer Weizman and other Israeli officials. Vance, national security adviser Zbigniew Brzezinski and other administration Middle East specialists are staying with Carter at Camp David.