The overland evacuation of Syrian and Palestinian forces along the Beirut-to-Damascus highway planned for Wednesday was called off today and apparently will be shifted to a sea evacuation from Beirut harbor, according to Israeli sources.

Israeli military officials announced tonight that the overland evacuation had been "postponed" at the request of Syrian authorities. They said they did not know the reasons for the postponement or when the forces that had been scheduled to evacuate via the highway between the two capitals would leave Beirut.

[Syrian official sources denied Tuesday night that they had requested the change, Agence France-Presse reported from Damascus.]

Informed Israeli sources said that they understood the evacuation had not been canceled or postponed but had been "shifted" to an evacuation by sea for reasons that remained unclear. The sources said those forces scheduled to leave Beirut by the overland route still would leave the city Wednesday on an extra ship chartered for the occasion and would be taken to the Syrian port of Latakia.

The last-minute change of plans was worked out among Syrian, Lebanese and American officials and was later agreed to by Israel, according to the Israeli sources.

According to the evacuation agreement, which was arranged by special U.S. envoy Philip C. Habib during two months of negotiations, the Palestinian forces and their Syrian allies in Beirut were to leave the city by either land or sea. A U.S. State Department fact sheet on the accord said the schedule was flexible and allowed changes as long as all groups agreed to the moves.

The overland trip to Damascus takes about four hours, but the trip by sea would be quite a bit longer.

Israeli Army officials speculated tonight that because of heavy fighting reported east of Beirut between Lebanese leftist Moslem forces and the Lebanese Christian forces, the Palestinians feared a Christian attack on the convoy carrying the Syrian and Palestinian fighters along the highway.

Other sources discounted this possibility, speculating that the Syrians were in effect "grandstanding" at the last minute out of anger at failing to prevent the election yesterday of the Maronite Christian commander of the Lebanese Forces, Bashir Gemayel, as president of Lebanon.

The forces who were scheduled to be evacuated by the overland route were regular Syrian Army troops and members of the Palestine Liberation Army, Palestinian fighters who are attached to Syrian Amry units. According to Israeli officials, under the evacuation plan for PLA forces were to evacuate across the Syrian border while the regular Syrian units would be allowed to redeploy behind Syrian lines in the Bekaa Valley of eastern Lebanon.

It was not clear tonight how many men were scheduled to evacuate Wednesday along the highway. In the past, Israeli officials have said there were 1,500 to 2,500 regular Syrian Army troops and 2,500 to 4,000 PLA fighters trapped in West Beirut.

The evacuation began Saturday and until now all of it has involved the movement of Palestinian forces out of Beirut by sea. Wednesday's evacuation was the first scheduled by land and the first involving forces other than PLO guerrillas.

Israeli military officials said that as of late tonight their troops had not become involved in the reported heavy fighting east of Beirut.

Prime Minister Menachem Begin said that the two-month-long war with the PLO in West Beirut was over, but he warned that Israeli forces will retaliate if attacked by Syrian and Palestinian troops still in eastern Lebanon, United Press International reported.

["We have no intention of attacking Syria or Jordan, but if we are attacked we will attack," Begin told the Foregin Affairs and Defense Committee of parliament, Israeli radio said.]