The first serious Israeli opposition to the Camp David accords erupted here yesterday as an ultranationalist group seized a hilltop in the occupied West Bank and established a "settlement" in defiance of the government.

While Prime Minister Menachem Begin is expected to return to a hero's welcome here Friday, yesterday's act of opposition indicated that he will also be met with a cacophony of political and emotional protests over the volatile issue of Jewish settlements in the occupied territories.

About 20 families of the rightist Gush Emunim faction erected a tent and a flagpole on the remote, almost inaccessible hilltop near Nablus to demonstrate what they called "the proper answer by the faithful of the land of Israel to the Camp David agreement."

The Israeli Cabinet, after an emergency meeting last night, issue a statement "repudiating" any settlements without government approval and said the squatters would be removed.

Troops were sent to the area but the steep, rocky terrain made it appear likely that the group could hold out for some time as a symbolic opposition to the agreements reached by Begin and Egyptian President Anwar Sadat.

While the Camp David accords do not deal with existing Jewish settlements on the West Bank, which the United States considers illegal, a key feature of the agreements is Egypt's prerequisite that Israel remove its settlements from the Sinai.

The Knesset will begin debating this issue Monday and although Israeli newspapers still carried full-page advertisements proclaiming, "Blessed are the peacemakers," pressure began building here yesterday from both ends of the politicial spectrum in the Knesset.

Gush Emunim, often with the implicit approval of the government, has been in the forefront of establishing illegal civilian outposts on privately owned Palestinian property. Its stated goal is to build Jewish settlements until Arabs are a minority in the West Bank.

Government officials did not say when the settlers would be removed, but the camp is located about two hours' climb from the nearest road, and authorities conceded it would be difficult to reach.

Asked if the demonstrators would resist arrest, a Gush Emunim spokesman said last night, "I don't know about resistance. I just know it won't be easy for the army to bring them down."

Besides having to deal with the demonstrators when he returns, Begin will also have ruffled feathers in the Knesset to smooth.

The opposition Labor Party agreed yesterday to consider a boycott of the Knesset debate - or abstain from a vote - if Begin presents the settlement question and the Egyptian-Israeli peace accord separately.

Party sources said that some members, fearing the onus of condemning 10,000 Jewish settlers to an evacuation from their pioneer communities in the Sinai, will insist that if Begin wants to remove settlements founded on the basis of a national consensus then he will have to present the motions as a package.

Accusing Begin of "emotional blackmail," the opposition sources said they would attempt to force Begin's Likud coalition to "show its cards."

With 30 of the Knesset's 120 votes, the Labor Party could cut deeply into the majority approval that Begin supporters expect.

At the other end of the political pole, there were increasing signs of defection from within Begin's own right-wing Herut Party, including several loyalists who served with the prime minister in the Irgun, the underground guerrilla organization during the British mandate of Palestine.

Said to be among then reportedly was Chaim Landau, a minister without portfolio, who reportedly was deeply disturbed at the prospect of voting to dismantle the settlements. If he voted to retain the settlements and the Sadat proviso was accepted, Landau presumably would resign from the Cabinet, following the tradition of collective responsibility for government action.

In 1970, Begin similarly resigned from the Cabinet, following the tradition of collective responsibility for government action.

In 1970, Begin similarly resigned from the Cabinet because he did not want to be associated with the ceasefire signed with Egypt after the war of attrition along the Suez Canal.

Also opposing an end to settlements were Knesset members Eitan Livni, former chief of operations of the Irgun; Geulah Cohen, a former radio broadcaster for the guerrilla group and Shmuel Katz, a longtime Herut Party comrade of Begin and a member of Israel's first Knesset.

The defections of some of Begin's closest political allies raised a striking parallel with the reactions by some of Sadat's inner circle, including Foreign Minister Mohammed Ibrahim Kamel, who resigned.

Begin, in a television interview shown here, acknowledged that he would have to "face a discussion with a number of my friends and colleagues" upon returning, but he insisted he felt he had "followed the right way."

Begin spoke fondly of the Gush Emumim, saying, "Even if they demonstrate against me and use bad language against me, I still love them. They are real pioneers. They will perhaps curse me, but I will return love.

Although it will not surface as openly, Begin is expected to hear opposition from Israel's military establishment, including complaints that removal of the settlements and abandonment of three airfields in the Sinai will compromise security.

While the United States has agreed to build two new Israeli airbases in the Negev Desert, some military analysts noted yesterday that these would be within artillery range of Jordan and would not present as effective a deterrent against Saudia Arabia's growing military strength as the Sinai bases.

Defense Minister Ezer Weizman, returning here yesterday with Foreign Minister Moshe Dayan, attempted to dispel that notion, saying that Israeli's army and air force will remain strong enough to guarantee national security.

Dayan, a leading Israeli war hero, said removing the settlements was "not an easy feeling, after having fought in the Sinai for so many years." But, in an airport news conference, he assured Israelis that removing them was necessary for peace.

He added, "We have the first chance in 30 years to reach a peace agreement, with diplomatic relations, an open Suez Canal, free passage to everybody, commerce, tourism . . . This is a chance we have been waiting and fighting for."

Dayan stressed that the question confronting the Cabinet will "not be when, but if" Israel evacuates the Sinai settlements.

"Just in case the Knesset decides to evacuate . . . then we shall negotiate the "when," Dayan said.

Weizman, in response to a suggestion that some Knesset members may want to vote for the peace agreement but against removal of the settlements, said at the airport news conference, "You can't separate one from the other. It's not so easy."

There was some confusion here yesterday over an apparent difference between Begin's and Carter's estimates of how long a proposed freeze on new West Bank settlement activity would extend. In Washington, Begin said no new settlements would be constructed on the West Bank for three months, while Carter indicated such building would cease for five years.

Dayan said the issue may be moot because "we do not plan to build any new settlements in the next few months."

Despite the internal squabbling he can expect in the weeks ahead, Begin still appeared yesterday to have broadened his political base in Israel as a result of the Camp David talks.

Even the most cautious Knesset head counts gave the peace proposal and the proposed evacuation of settlements a slim majority if submitted separately and a wide margin if submitted as a package.

Begin can be expected to reassess the climate once he arrives, but his position seemed so strong that there were reports here - denied by his aides - that he is confident enough of success to be thinking about advancing elections to strengthen his position in the parliament.

Israel's newspapers warmly praised Begin in editorials yesterday - not an everyday occurance for him.

Haaretz, the independent morning paper, said Begin's "undertaking that there should be no more new settlements during the negotiations is a concession to which he was firmly opposed so long as it was advocated by the Israeli opposition, and the more appreciated for this reason."

Davar, affiliated with Histadrut, the national labor union, editorialized, "after 11 months of talks alternating with crisis, launched by Sadat last November, Mr. Begin was faced with the choice of either signing the agreement or causing the conference to end in a fiasco. He chose to avoid such a fiasco, and he took the right decision."