Intense sectarian fighting erupted in the mountains overlooking Beirut early today as Christian and Druze militias battled for control of key positions abandoned by occupying Israeli forces.

Militiamen dueled with rockets, mortars and artillery in the mountains. The Druze shelled Christian east Beirut continuously from the afternoon into the early evening. Ground battles appeared aimed at gaining control of the vital Beirut-to-Damascus highway.

Stunned by the sudden withdrawal and the Druze attack, the Lebanese Forces Christian militia issued a communique saying, "Our forces are holding fast against the attackers, waiting for the Lebanese Army to deploy in Israeli-vacated areas."

But the Lebanese government, according to several sources, made a "political decision" not to deploy the Army in the mountains today, apparently hoping that frenzied negotiations could result in an accord allowing the Army to enter without getting caught in the fray.

An armored column of Army soldiers did move to take control of the stretch of the coastal highway, south of the capital to the Awwali River, that was abandoned by the Israelis in the pullback. Army tanks easily rolled over token Druze opposition on low ground but were the targets of heavy shelling from Druze positions in the mountains. They returned the fire.

A U.S. marine, apparently caught in the crossfire of that fight, suffered a slight shrapnel wound in his left arm when mortar fire fell near marine positions this afternoon. He was treated and returned to duty.

Marine spokesman Robert Jordan said that at one point during the ground fighting between the Lebanese Army and Druze militia south of the airport, marines fired rifles at militiamen who had shot at them.

Predominantly Moslem west Beirut and the slums south of it were tense. Lebanese officials indicated concern that militiamen there might link up with the fighting in the mountains.

There was also worry this morning that Syria might enter the conflict on the Druze side, adding a perilous new dimension. But those fears subsided later in the day.

The Israeli military command in Tel Aviv said two Israeli jets "strafed" tanks that moved from Syrian-controlled positions in the mountains to attack Christian Phalangists. The communique did not say whether the tanks were Syrian or those of Druze militiamen. Christian Phalangist radio and militia spokesmen here said they were Syrian but Moslem allies of the Druze in Beirut insisted they were manned by Druze.

Israeli Defense Minister Moshe Arens said later at a press conference in Tel Aviv: "The major contenders in the fighting that has been going on today are the Lebanese Forces and the Druze militias in the area. It is possible that one or both these bodies have received assistance here or there in terms of equipment from other sources, as for example the Syrians. But to the best of my knowledge there is no direct Syrian involvement in the fighting at the present time."

Arens, discussing negotiations prior to the pullback, said: "We made it clear that we do not want hostile forces entering the areas we leave . . . . I think that message is well understood in Syria."

Amid the thunder of the heavy artillery, there was bloodshed and acrimony here. Red Cross ambulances raced through the neighborhoods of east Beirut, their flags flapping, to carry off casualties.

Police said 34 people were killed and 71 wounded in the mountains and east Beirut, The Associated Press reported.

At least nine persons were reported killed and as many injured in a car-bomb explosion in west Beirut.

Lebanese newspapers accused Israel of a "double cross" and a formal Army communique complained that the partial withdrawal was carried out without notice or coordination. Lebanese state radio said the government of President Amin Gemayel had declared a tentative accord reached with Israel May l7 "as good as frozen," although it was unclear what that meant in practical terms.

The Israeli Defense Forces, in a communique issued today, said, "Contacts between the IDF and the representatives of all sides are going on continuously during the pullback. Throughout the night efforts and arbitration attempts were continued by IDF officers at all levels, for the purpose of bringing about an agreement between the Lebanese government, the Lebanese Forces and the Druze. However, all attempts failed and no agreement was reached.

"The Lebanese Army, whose entrance to the area was conditional upon an agreement being reached, made absolutely clear that it has no intention of entering the area being vacated by the IDF."

Israel had delayed its pullback, at the request of the United States, to give the Lebanese time to work out their complex differences. When Israeli soldiers finally left early this morning, a spokesman for the Israeli Army said wearily that he was not certain the sectarian battles in Lebanon would ever end.

U.S. military officers here said the Lebanese had to be aware yesterday that the Israelis, who have been thinning out forces in the mountains for more than a month, were withdrawing when they saw fuel trucks and other essential logistical gear moving out.

A Lebanese Army communique said arrangements for a gradual deployment in the Chouf mountains above the capital were being made "in light of developments and situations required by the country's interest." But it did not specify when.

The indications were that frantic efforts, with Saudi Arabia attempting to act as mediator, were being made to reach accommodation between the government and Druze leader Walid Jumblatt--who has threatened a "bloodbath" if the Army is deployed in the mountains before political accord is reached.

U.S. officers said it was clear that both Druze and Christian fighters were weakening under the force of their mutual artillery exchanges. A Lebanese Army sergeant said, "I hope the Army will let them fight each other for a while so they both will step down."

Others here worried, however, that if the mountain battles are not quelled soon, the trouble might soon spill over into the capital.

The uprising of Moslem militiamen that swept west Beirut last week was touched off by a relatively minor incident in the aggrieved southern slums and only put down after an armor and infantry advance into that part of the capital.

Even as Israeli forces pulled out of the mountains early this morning in long convoys of tanks and armored personnel carriers, what a Phalangist spokesman described as a "free-for- all" broke out in an effort to take over the evacuated positions.

Along the coastal highway, a squad of Druze militiamen initally claimed a vital intersection at Khaldah, which they held until the Lebanese Army armor rolled through in the afternoon. As the militiamen checked passing motorists this morning, another driving a tractor attempted to dig up dirt from a traffic island for embankments to halt the Army advance. About a half mile south, Christian militiamen claimed another portion of the coastal road.