A group of Roman Catholic scholars and activists who signed a controversial advertisement on abortion branded as a "scandal" yesterday Vatican threats to expel 24 nuns who signed the statement.

"We believe that this Vatican action is a cause for scandal to Catholics everywhere," said a statement issued by between 35 and 40 of the 97 people who signed the "Catholic Statement on Pluralism and Abortion," which appeared as a full-page ad in The New York Times on Oct. 7.

The ad, noting that recent popes and bishops have condemned abortion "as morally wrong in all instances," said that "there is the mistaken belief in American society that this is the only legitimate Catholic position. In fact, a diversity of opinions regarding abortion exists among committed Catholics."

Yesterday's gathering was in response to a letter last month from the Vatican's Sacred Congregation for Religious and Secular Institutes directing the 24 nuns to retract the statement or face possible dismissal from their orders. Two priests and two religious brothers also are subject to discipline.

Those who met yesterday, who included lay people as well as some of the affected nuns, said they were "appalled" by the Vatican's action, which they said "seeks to stifle freedom of speech and public discussion in the Roman Catholic Church and create the appearance of a consensus where none exists."

One signer of the ad, who asked not to be identified, said it was prompted by the widely publicized views on abortion of New York Archbishop John J. O'Connor during the fall election campaign.

In what became a running battle with Democratic vice-presidential nominee Geraldine A. Ferraro and New York Gov. Mario M. Cuomo, both Catholics, O'Connor held that public officials who are Catholic have an obligation to work for laws outlawing abortion.

In its statement yesterday, the group charged that the action "seems to be another attempt by the Vatican to silence public discussion in the church, whether voiced by theologians of the First or Third World, bishops, clergy and religious holding public office, especially women in the church."

"The current Vatican action requires women to comply with the directive of a patriarchal system in which they have no real voice nor power," the statement continued.

Calling the crackdown "our season's greetings from the Vatican," the group urged people to send Christmas cards to the Sacred Congregation for Religious and Secular Institutes "stating your belief in diversity, pluralism and honest discussion of church issues."

Letter-writing to the Vatican has been used with telling effect in recent years by some Catholic groups in the United States. Publications regularly include names and mailing addresses of Vatican officials and offices to which protests may be sent.

Yesterday's statement also expressed "surprise and anguish that the Vatican could so readily suggest the dissolution of what the nuns consider a profound and mutual commitment." It called for "a just and nonviolent resolution" of the dispute.

One of the nuns who signed the ad, Sister Maureen Fiedler of Mt. Rainier, Md., said, "Many of us are struggling with the question of wanting very much to stay in our religious communities and wondering how we do that and retain our integrity."

In their prepared statement and in comments to reporters after their meeting yesterday, group members made clear that they were speaking only for themselves.