On a beach near where an Israeli amphibious force landed 15 months ago, a long line of Israeli armored personnel carriers filled with happy soldiers waited today to be loaded onto ships for a trip home that closed one chapter of Israel's involvement in Lebanon and opened another.

The seaborne evacuation was but one part of Israel's rapid and apparently smooth evacuation of the positions it has held in Lebanon since last summer to a new line along the Awwali River just north of here.

The Israelis called it Operation Millstone, a pointed reference to how they viewed being caught in the crossfire of Lebanon's bitter sectarian rivalries that were further inflamed by the Israeli invasion and its aftermath.

Beginning in darkness at about 10 last night, protected by air cover and fresh combat units that were brought in for the withdrawal on the ground, the Army moved south along the coastal highway and two other routes farther east, peeling away from the outskirts of Beirut, the Beirut-to-Damascus highway and the Chouf mountains southeast of Beirut.

By dawn today, according to Army officials, the bulk of the Israeli Army was already out of the strife-ridden Chouf region and heading for the Awwali. By midmorning, the coastal highway between Damur and Sidon was virtually deserted, most of the Army's heavy equipment having already passed.

But just south of Damur, the sound of artillery apparently being fired by rival Christian and Druze militias in the Chouf mountains could be heard.

Army officials said there were no reports of casualties or injuries in connection with the withdrawal and that fire had not been directed at the Israeli troops.

Army chief of staff Lt. Gen. Moshe Levy landed by helicopter at Damur at about 6:30 this morning and said the withdrawal "is going as planned and a little bit faster." Levy said the pullback could be completed by tonight, and by that time it appeared to be finished.

At a news conference in Tel Aviv tonight, Defense Minister Moshe Arens said the withdrawal "was well planned and well executed and, at least so far, carried out without a hitch."

Israeli radio reported that between 100,000 and 150,000 Lebanese had fled south behind the new Israeli lines in the last few days to escape the anticipated outbreak of new fighting between the Christians and Druze militias. This was said to have taken place mostly in central and eastern Lebanon.

To the west, there was no evidence today of civilians traveling south along the coastal highway, which the Israelis closed to all but their own military traffic from just south of Beirut to the Israel-Lebanon border.

The Israelis had ample time to plan the withdrawal, which appeared to take place with precision. Beginning last night and continuing through this morning, hundreds of Israeli trucks crossed the border and streamed into Lebanon, just as they did on June 6 last year at the outset of the invasion.

But today the heavy trucks were empty and heading for staging areas such as one near the Zahrani River south of Sidon where tanks and other equipment were to be loaded on them to be hauled farther south.

The new Israeli line on the Awwali River is about four miles north of Sidon where a main checkpoint was in operation today.

Typical of how the pullout went was the experience of an Israeli unit stationed near Alayh. The first convoy of water tankers and support equipment left the area late yesterday afternoon.

The rest of the unit was to move at daybreak today, but after a change in orders that may have been linked to a reported last-minute appeal to delay the pullback, the unit was on the move six hours earlier than scheduled.

The soldiers attached pale green lights and bright orange roof coverings to their vehicles to make them easily identifiable to the overhead Israeli jets. They were accompanied by Israeli border policemen.

As the withdrawal was going on according to schedule, government officials in Jerusalem defended the Israeli decision to pull back now rather than waiting longer for a possible agreement to prevent renewed fighting between the Christians and Druze.

"One more day or one more week, if there would have been a chance we would have waited," said a senior Israeli official. "But I don't see how it will be different a year from now . . . . We didn't see it an accord coming about."