More than 3,100 diplomats, State Department employes and ordinary citizens packed the main nave of the Washington Cathedral yesterday for midday memorial services honoring the 48 Americans and Lebanese killed in last week's explosion at the U.S. Embassy in Beirut.

Vice President George Bush, dozens of other dignitaries, and relatives of some of those killed, heard Acting Secretary of State Kenneth Dam declare that the victims' lives were lost in the search for peace--a search, he said, terrorism would never be permitted to halt.

"Ours is the grand experiment in freedom which offers hope to all mankind," Dam said. "Let it therefore be known that we will not be deterred by cowardly acts of terrorism. Peace cannot be killed by killing the peacemakers."

The 17 Americans who died, Dam said, "were among the best of us" and should remind the nation "that the ideals of patriotism, honor and service still endure."

"If we fall into the trap of cynicism, their example of devotion to duty will free us," he said. "If we grow tired in our pursuit of peace, their courage and perseverance will inspire us. And if we become callous in the face of suffering, their sacrifice will humble us."

Employes from the State Department, Central Intelligence Agency and Defense Department, crowded into the vast, Gothic cathedral for the 40-minute service that was also attended by Secretary of Defense Caspar Weinberger and the diplomatic corps led by Soviet Ambassador Anatoliy Dobrynin.

The following Americans have been listed by the State Department as having died in the embassy explosion:

Robert C. Ames, 49, Director of Near East and South Asian Analysis for the Central Intelligence Agency.

State Department employes: Phyliss N. Faraci, 44, administrative specialist; Terry Lee Gilden, 26, of Washington State, on temporary duty as security officer; Kenneth E. Haas, 38, political officer; Deborah M. Hixon, 30, foreign affairs analyst; Frank J. Johnston, 47, of New York, economic officer; James Foley Lewis, 39, political/military officer; Monique Lewis, 36, part-time secretary and wife of James F. Lewis.

Defense Department employes: William Richard Sheil, 59, civilian employe of the Army; Army Staff Sgt. Ben H. Maxwell, 26, of Appomattox, Va., equipment maintenance; Army Master Sgt. Richard Twine, 36, of Salop, England, equipment maintenance; Army Staff Sgt. Mark E. Salazar, 30, of San Gabriel, Calif., equipment maintenance training; Marine Cpl. Robert V. McMaugh, 21, of Manassas, Va., security guard.

Agency for International Development employes: Thomas R. Blacka, 59, of Denver, AID controller; William R. McIntyre, 52, of McLean, deputy director of AID mission to Lebanon; Albert N. Votaw, 57, of Chester, Pa., AID Housing and Urban Development officer.

Janet Lee Stevens, 32, of Venice, Fla., Beirut correspondent for Asahi Shimbun of Japan.