Roman Catholics throughout the country marked the anniversary of the murder of El Salvador Archbishop Oscar Romero yesterday with prayers and vigils, and a growing determination to fight the Reagan administration's escalating military aid to that country.

With the possible exception of abortion, no social issue in recent times has so engaged Catholics in this country as the controversy over El Salvador, where four American women missionaries also were murdered last December. Catholics here support Romero's plea that Salvadorans be allowed to determine their own destiny and correct abuses that have kept so many in abject poverty.

Yesterday's anniversary of the death of Romero, who was gunned down while saying mass in a hospital chapel, provided the occasion for Catholics in this country to demonstrate their feelings. In addition to hundreds of memorial services, there were special lessons in parochial schools, special vigils, and protest demonstrations at such sensitive spots as the White House and State Department, all of which reflected and reinforced the deep concern of Catholics over the issue.

"We don't have any way of knowing exactly, but I have the sense that there is hardly a diocese that isn't somehow doing something," said Thomas Quigley, Latin America expert for the U.S. Catholic Conference. "They are doing things in all sorts of places that are not normally on the map of social protest." He emphasized that the hierachy neither organized nor kept records on such activities.

At St. Matthew's Cathedral here yesterday, some 500 worshippers at the noon mass unexpectedly interrupted Auxiliary Bishop Thomas W. Lyons with prolonged applause when he said, during his tribute to Romero, "I think we can deny the implication that if we oppose providing weapons to [El Salvador] we are somehow allied with the communists."

Lyons, who usually maintains a low profile on social issues, named no names. By many in the congregation viewed his comment as a reply to charges by John A. Bushnell, acting assistant secretary of state for inter-American affairs, that, within Latin America at least, protests against U.S. policy were the work of a "well-orchestrated effort" by a "worldwide communist network."

"That's our answer to Bushnell," said one priest of Lyon's comment, in remarks after the mass.

In opposing military aid to El Salvador, U.S. Catholics have followed the unusually forceful lead of their bishops, who have testified before Congress, called at the White House and supported telegram and letter-writing campaigns. Even more effective in raising the consciousness of church members have been the networks of nuns and priests, many of whom have served in Latin America.

The El Salvador situation has spawned numbers of ad hoc education and action organizations, some national, some local, many of them including representatives of mainline Protestant Churches which are beginning to join Catholics in protesting U.S. policy.

The president of the National Council of Churches, the Rev. M. William Howard, has written the heads of the 32 communions that make up his organization alerting them to stand by for a "significant campaign" during Holy Week "which may require your physical presence in the nation's capital," it was learned.

El Salvador has cut across liberal-conservative polarizations within the Catholic Church as few issues have. "I have a sense that it has penetrated the country in a way that has not happened before," said Sister Ann Gormly of the U.S. Catholic Mission Council, and is approaching the depth of concern that the right-to-life question calls forth.

"We've never gotten that kind of concern for a justice issue before," she said. "I think it was probably the martyrdom" of the missionaries, three nuns and a laywoman, "which has involved people who are not normally affected" by political issues.