Episcopal Bishop John T. Walker yesterday called on governments of the free world to break off diplomatic ties with Uganda in order to force the downfall of Ugandan President Idi Amin.

Speaking at a memorial service for Ugandan Archbishop Janani Luwum held at the Washington Cathedral, Bishop Walker called upon "those of us who care" to "demand that governments who honor this tyrant (Amin) cease to do so . . . that embassies be closed so that he may know that the world does not approve of what he does."

Bishop Walker echoed sentiments expressed in even stronger terms at an ecumenical memorial service attended by several thousand worshippers in Nairobi, the headquarters of the All-Africa Conference of Churches.

Kenya Methodist Bishop Lawi Imathiu said, "The church has been silent too long . . . It is all very well to condemn white regimes and turn a blind eye elsewhere, but the time has come for the church to be a church, otherwise we are doomed."

After yesterday's service here, attended by nearly 500 people, Bishop Walker suggested the possibility of some international Christian action against the Amin government. Consultation is already under way, he said, between the archbishop of Canterbury and Presiding Bishop John M. Allin of the Episcopal Church in this country. In addition, Canon Burgess Carr, head of the All-Africa Conference, is expected here shortly.

Uganda officials said Archbishop Luwum and two senior cabinet ministers were killed Wednesday in an automobile accident following their arrest in an alleged plot against Amin. But both church and government leaders outside Uganda, including United Nations Ambassador Andrew Young, have branded the deaths as political assassinations.

Bishop Walker yesterday read a brief message from the White House, which supported Young's statement and said if "reflects the concern and reaction of Americans to the deaths of the archbishop and the two government officials."

Ambassador Young on Thurday had called the deaths "sadistic and malicious actions that need to be condemned."