Thousands of Ethiopian exiles will be allowed to remain in the United States indefinitely under a change in policy being prepared by the Immigration and Naturalization Service, officials said yesterday.

Most of the exiles arrived in the chaotic wake of a Marxist takeover of the Ethiopian government in 1974. They had been notified to appear for deportation hearings beginning last August, when the Reagan administration decided the Ethiopian government had moderated so that it would be safe for the exiles to go home.

INS officials estimated that 3,000 to 4,000 people were likely to be deported, while Ethiopian refugee group leaders put the number at 15,000 or more.

But opposition to the deportation proceedings arose from an unusual ad hoc coalition of Ethiopian refugee and human rights groups, liberal members of the Congressional Black Caucus and conservative members of Congress worried about repression of anti-Marxist dissidents.

They got unintentional help from U.N. Ambassador Jeane J. Kirkpatrick, who said in an October floor statement that the Ethiopian government was "one of the most oppressive . . . in the world," even though the State Department was saying the opposite.

The opponents of deportation produced Ethiopian government documents purporting to show a policy of imprisonment for critics and won stiffly worded resolutions from both the Senate and the House asking that the exiles be permitted to stay.

The State Department reconsidered, and earlier this week wrote the Justice Department, which includes the INS, advising that Ethiopians here before Jan. 1, 1980, should be allowed to remain. "We're reviewing our policy and will be coming out with a new one very shortly," INS spokesman Duke Austin said yesterday.

The letter, from Deputy Secretary of State Walter J. Stoessel Jr. to Attorney General William French Smith, recommended that none of the earlier Ethiopian refugees be forcibly returned to Ethiopia, according to sources close to the proceeding.

The officials said that means that most of the refugees will be allowed to retain the "extended voluntary departure" status they were given on arrival, which in practice allows them to stay as long as they like.

The estimated 3,000 Ethiopians who arrived during 1980 and 1981 came after their government was more carefully controlling departures, so their cases will be dealt with on an individual basis rather than receiving the blanket clearance, the sources said.

Architects of the move included Elliott Abrams, assistant secretary of state for human rights and humanitarian affairs, and Reps. Julian C. Dixon (D-Calif.), a liberal Congressional Black Caucus member, and Jack Kemp (R-N.Y.), a tax-cutting conservative.

"We put together a few little task forces . . . that called on every House Judiciary Committee member who would see us," related Carolyn Waller of the alien rights project of the Lawyers Committee for Civil Rights Under Law. The eight task force groups, made up of one or two Ethiopians and one or two human-rights group members, saw 20 of the 27 committee members, she said, adding that strong support came from at least one member the groups did not see, Rep. Hamilton Fish Jr. (R-N.Y.).

Kemp said he had received "unofficial notification" of the policy change and added that he is "happy with this very positive response from the State Department."