The battle between Christian and Moslem militias for the ancient port city of Sidon raged on yesterday, with Syria reportedly preparing to step in before Lebanon's warring factions are swept into full-scale civil war, The Associated Press reported.

Officials said a policeman was killed and 15 civilians were wounded in exchanges of rocket-propelled grenades and mortar and machine-gun fire between Christian militias on one side and Moslem militiamen and Palestinians on the other. The Christian fighters are loyal to rebels who, objecting to Syrian influence over Lebanese President Amin Gemayel and the president's efforts to give Moslems more power, last month took control of the main Lebanese Christian forces in Beirut and regions north of the capital.

Yesterday's fighting brought the casualty toll to 48 killed and 194 wounded during eight days of conflict in southern Lebanon's largest city, 25 miles south of Beirut. There was less intense fighting -- and no casualties reported -- in other areas.

In Jerusalem, Prime Minister Shimon Peres rejected charges by France, the United Nations and the U.S. State Department that Israel had violated the Geneva Conventions by transferring about 1,200 Shiite Moslem prisoners from Lebanon to Israel.

Peres said the inmates, who were moved from the Ansar prison camp in southern Lebanon to an undisclosed camp in Israel, are not prisoners of war, and therefore are not covered by the Geneva Convention stipulation that civilian prisoners in occupied territory not be moved from their country.