Israeli tanks rolled down the Beirut-to-Damascus highway in long convoys this morning as the long-planned withdrawal from the Chouf mountains got under way.

Israeli spokesmen here said they expected the deployment to be completed by this afternoon. They shrugged aside the alarms of Lebanese that the redeployment had not been coordinated with the Lebanese Army and might lead to a flare-up of civil conflicts here.

"At this stage in time we have no reason to believe that if we gave them any more time, that anything will change," said an Israeli spokesman.

In Washington, U.S. officials said special U.S. envoy Robert C. McFarlane, who had come to Washington Friday to brief President Reagan and his top advisers, would go back to Lebanon Sunday.

The partial pullback, which Israel announced in July and delayed at the request of Lebanon and the United States, will remove Israeli troops from the suburbs of Beirut and the Chouf mountains, scene of fighting beween Lebanese Christians and Druze militias since last fall.

The thundering sound of illumination rounds to light the Israelis' way could be heard as Israeli soldiers moved down the highway in tanks and armored personnel carriers this morning. Spokesmen said the heavy equipment began moving at midnight, although sources here say the Israelis have been withdrawing logistical support units for several weeks.

Israel will establish a new front line at the Awwali River near Sidon, 20 miles south of Beirut, but keep control of strategically important mountain ranges near the Beirut-to-Damascus highway and portions of the Bekaa Valley.

With the pullback, in which it will relinquish about 240 square miles of the territory it occupied in its invasion of Lebanon last year, Israel hopes to be able to remove up to a third of the 20,000 troops it still has in Lebanon and to reduce the number of casualties its occupying Army has been suffering from hit-and-run attacks--a situation that has created considerable political pressure for the Israeli government.

As residents of the mountain towns south of Beirut reported convoys of Israeli troops moving out, Lebanese, U.S. and Israeli officials expressed concern about the possibility of greater sectarian bloodshed in the area being evacuated.

Uri Lubrani, Israel's coordinator in Lebanon, told Israeli radio, "We are very, very concerned. A diverse and complex effort is being exerted to get an agreement to avoid bloodshed, massacres and shooting."

Lebanese state radio said that "contacts are being made at the most senior levels to prevent a return of tension and fighting in the mountains."

Lebanese President Amin Gemayel summoned Prime Minister Shafiq Wazzan and other top officials to a meeting at the presidential palace. Wazzan told reporters, "We are heading off to pursue talks on this fateful day since the Israelis, according to our information, are preparing to go ahead with a partial withdrawal despite our stated policies."

McFarlane had been attempting to forestall an Israeli pullback until the Lebanese government was able to get an agreement between the warring Christian and Druze to allow the Lebanese Army to replace Israel's presence in the Chouf mountains.

The Associated Press reported from Tel Aviv that Israeli Defense Ministry spokesman Nahman Shai said Defense Minister Moshe Arens had met with U.S. envoy Richard Fairbanks, McFarlane's deputy, and told him, "Israel's Army can no longer delay its redeployment." Shai said the two men met in the town of Dahr al Kamar in Lebanon's Chouf mountains. Israeli television quoted Arens as telling Fairbanks that Israel "regrets that the Lebanese government has not made more efforts to come to agreement with the militias in the Chouf."

The Lebanese government has announced it is determined to deploy three brigades of U.S.-trained Lebanese soldiers in the mountains, whether or not there is national political accord beforehand.

Druze leader Walid Jumblatt, backed by Syria and issuing his pronouncements from Damascus, has threatened a "bloodbath" if the Army does so. And yesterday Druze militiamen threatened "endless war" if Lebanon's Army attempts to move into Druze villages in the Chouf mountains to exercise the control that Israeli troops had exercised in the past and try to halt fighting between Druze and Christian Phalangist forces.

Privately owned radio here said artillery battles broke out yesterday afternoon between Christian and Druze militias, but it was not immediately clear whether they were related to an Israeli withdrawal or just another of the battles that have raged frequently.

Reagan administration officials said late yesterday that the Gemayel government was negotiating "very actively" with the Druze and other opposition groups to pin down an agreement allowing the Lebanese Army to move in in the wake of the departing Israelis. But as of late afternoon, U.S. officials said, no agreement had been reached, and the Army had not started to move into the Chouf.

U.S. officials in Washington said privately that McFarlane reported that Syria continues to be "an adverse influence" in Lebanon, largely through its support of the Druze militia.

The first priority for the United States, they said, is to help Lebanese armed forces take control. But they said there are no plans at this time for any changes in the size or mission of the multinational peace-keeping force, made up of 1,200 U.S. Marines and troops from Britain, France and Italy.

The Israelis advanced into the Chouf mountains last summer in the invasion of Lebanon, and the Christian Phalangists, who were then their allies, followed them. While the Israeli rout of Palestinian fighters was unopposed--and often welcomed--by both Christians and Druze, the Christian militiamen's arrival aroused old hatreds that had lain dormant during seven years of civil war in Lebanon.

Israeli forces have been accused by Lebanese, western diplomats here and Israeli journalists of aiding both sides in the sectarian warfare in the mountains, which has included artillery duels, firefights, snipings, kidnapings and retaliatory murders of civilians.

Israeli officials have consistently denied the allegations although their presence in the mountains has caught them in the middle of two allies.

Israel trained and sold arms to the Christian militia and sought to bring them to the position of dominance in Lebanon as part of its strategy in last summer's invasion. Druze in Israel form an influential political bloc and they have made strong appeals to the Israeli government to protect their coreligionists in Lebanon's mountains.

Meanwhile, the Lebanese government deployed 2,000 Army troops in Christian east Beirut before dawn today in an effort to maintain the calm that has existed during the past three days in the capital.

Moslem leaders here had insisted that the Lebanese Army take firm control of Christian east Beirut following its armored blitz Wednesday into west Beirut to crush an uprising of previously underground Moslem militias.

The Army was unopposed as its tanks rolled into east Beirut yesterday morning but it remained unclear whether the Christian Phalangist militia there would vacate offices and barracks and cease collecting taxes, as Moslems in west Beirut are demanding.

Fears that fighting might quickly resume were exacerbated by concerns about the Israeli withdrawal from the mountains south of Beirut. There was deep concern here that if no political accord is reached before the Israeli pullout from the mountains, fighting there could sharply escalate and link up to the bubbling turmoil in the capital.

Although the Lebanese Army's advance into west Beirut on Wednesday quieted snipers and got masked militia gunmen off the streets, officials here believe there are still large stores of weapons hidden there.

Adding to concerns is the continued existence of armed Shiite Moslem militiamen in the slums on the southern outskirts of the capital around Beirut International Airport, where U.S. Marines in the multinational force have positions.

Lebanese officials fear that these militiamen will assault the Army from the rear as it deploys in the nearby mountains, forcing the fledgling government forces to battle on two difficult fronts.

While few expect that Jumblatt can be persuaded soon to come to terms with the government before the deployment in the mountains, the hope here is that an accommodation might be reached with the Shiites in the coastal slums.

Washington Post staff writer John M. Goshko contributed to this report in Washington.