American Catholic nuns yesterday marked the first anniversary of the murder of four women missionaries in El Salvador with demonstrations here and elsewhere around the country calling for a U.S. foreign policy keyed to self-determination and social justice in Latin America.

About 200 women and a sprinkling of men from across the country clustered on the east steps of the Capitol during the noon hour yesterday to remember their slain comrades, sing hymns, and listen to their leaders denounce American military aid to what one termed "the oppressive government of El Salvador."

Then they fanned out to lobby members of Congress and reassembled again last night, with other demonstrators, across from the White House to bring their message to the executive branch.

The occasion for the day's protest was the slaying a year ago of Maryknoll Sisters Ita Ford and Maura Clark of New York, Ursuline Sister Dorothy Kazel of Cleveland and Jean Donovan, a laywoman from Stamford, Conn. The women were last seen alive shortly before being stopped at a national guard roadblock near the San Salvador airport.

Six national guardsmen have been arrested, on the basis of evidence developed by FBI investigators, but there has been no move to bring them to trial. Salvadoran junta president Jose Napoleon Duarte has said prosecutors lack sufficient evidence to try them.

Catholics here and in El Salvador, where memorial masses were celebrated for the slain women, are convinced that El Salvador government forces bear some responsibility for the deaths of the four Americans and thousands of Salvadorans. Catholic leaders in this country steadily have echoed the appeal of San Salvador acting Archbishop Arturo Rivera y Damas that the United States withold military aid from the San Salvador government.

"We grieve not only at the brutal slaying of these four exemplary women," said Sister Catherine Cafferty of North Dakota yesterday at the vigil on the Capitol steps, "but more deeply, we grieve that our government cannot escape the responsibility" for the many deaths in El Salvador.

The protests and vigils yesterday were coordinated by the Leadership Conference of Women Religious, an umbrella organization including 80 percent of orders of Catholic nuns nationwide. They were part of a growing effort by nuns and orders of priests and brothers, many of whom have first-hand missionaries ties to Latin America, to persuade U.S. government policymakers to make respect for human rights a criterion for U.S. aid.

A secondary theme of events yesterday, which members organized in the conference's 15 regions across the country, was opposition to nuclear armament.

Sister Joan Carusillo of Washington summed up the political agenda of the group yesterday. "We join with our sister and brothers across the nation who are publicly opposing the escalation of the nuclear arms race," she said.

On El Salvador, she reiterated the position of the American Catholic bishops, with whom "we stand in solidarity": end American arms shipments to El Salvador, stop deporting Salvadoran exiles who have taken refuge in this country and "admit that the situation in El Salvador is the continuation of a 10-year struggle of poor people to win for themselves the right to determine the destiny of their tiny country."

U.S. Ambassador to El Salvador Deane R. Hinton has hinted that he doubts the killers can be convicted because strict El Salvador rules of evidence require virtual eyewitness testimony for a conviction in murder cases.