A gathering of 250 people -- some mournful, some prayerful, many optimistic -- assembled yesterday morning at Sheridan Circle in Northwest Washington to observe the fifth anniversary of the murder of Orlando Letelier and Ronni Karpen Moffitt.

Letelier, a former Chilean diplomat in exile from his homeland and a fellow for two years at the Institute for Policy Studies, and Moffitt, a 25-year-old former teacher and institute coworker, were killed by a bomb while motoring through the circle to work on Sept. 21, 1976.

Murder charges still are pending against three senior officials of the Pinochet government in Chile, though neither that government nor the Reagan administration is pressing the case. Two anti-Castro Cubans, who intially had been convicted of murder and conspiracy charges in the case, were acquitted by a jury in a retrial on the same charges last May. A third Cuban still must stand trial on lesser charges in the case while two other Cubans remain fugitives.

The focus of yesterday's ceremony was the unveiling of a monument memorializing Letelier and Moffitt. The small circular structure, which contains a bronze disk featuring the sculpted faces of the two, is located next to a ginkgo tree on the sidewalk on the outer south rim of the circle and bears the dates of their births and death. The memorial's designer was Ned Echeverria, a Washington urban planner.

"It is fitting that we dedicate this momument on public ground," said Marcus Raskin, one of those responsible for hiring Letelier and Moffitt at the institute, a Washington research organization. "For these two were public people who laid out their lives and their blood in the improvement in the lives of others . . . They sought justice, serving it, even suffering injustice rather than do an injustice to others. That was, I suppose, what was so maddening to those who ordered the assassination."

Among those placing flowers around the new monument were Isabel Letelier, the widow of the slain diplomat and who lives in Washington and directs the Committee for Human Rights in Chile; Hilda Karpen, the mother of Ronni, and her two sons Harry and Michael Karpen; Jacobo Timerman, the former Argentine political prisoner and author of "Prisoner Without A Name, Cell Without A Number;" and Sister Blaise Luppo of the Maryknoll Sisters.

Later in the day, at a luncheon at the Georgetown Hotel, Timerman and Luppo received the 1981 Letelier-Moffitt Memorial Human Rights Awards.

Richard J. Barnet, another member of the institute, told those gathered for the ceremony that "on this day when we remember a personal tragedy on the loss of friends, we also remember the struggle that goes on in the world for human rights and justice . . . "

Stressing the need for a strong human rights policy, Barnet said, "It is a tragedy that in this city and at this time, and with the suffering of the world as it is, that the priorities seem otherwise . . . Those who would try to build a politics predicated on the abandonment of people are seen now to be in the ascendancy -- here, in Chile and in other places in the world."

After the ceremonies, as Mrs. Letelier and Mrs. Karpen moved among friends and strangers to thank them for coming, Timerman, standing near the flower-covered monument, was asked about the feelings of the crowd.

"I don't know how to analyze feelings." he said. "I'm optimistic. Optimism is not a feeling, its a political decision."