A heavy steel gate stretching across a narrow mountain road here 15 miles north of Israel's border with Lebanon marks the northern end of Israel's new security zone inside southern Lebanon. In a symbolic sense, the gate also will soon mark the start of a new era in Israeli-Lebanese relations.

The gate was put in place across the road running north to Jezzin a month ago, part of the feverish preparations that have been taking place as Israel nears the moment when it will end its three-year occupation of southern Lebanon and the last Israeli Army units still here will pull back to the border.

That moment is expected to come by this weekend, and perhaps as early as Thursday night, and will be noted by a simple announcement from the government in Jerusalem.

Israel's last hours in Lebanon have been nothing like the first, when the Army stormed across the border in full force. Most Israeli units already have been withdrawn from Lebanon. Those that have not, like the soldiers who stood on a hillside several miles south of here today watching their vehicles and equipment being loaded onto flatbed trucks wait impatiently for the order to move out.

The withdrawal has been going on in stages since mid-February and by now has been drained of much of its drama. But when the last units cross the border into Israel, it will mark the start of a new era in Israel's relations with Lebanon, just as the June 1982 invasion was the start of a military adventure, now ending, that a majority of Israelis have come to see as far too costly and ultimately unsuccessful.

At the outset at least, it will be an era much like the four years that preceded the invasion, when the Israelis relied on surrogate Lebanese militia forces backed by Israeli supplies, financing and advisers to control the strip of land along the border that now falls within an enlarged security zone.

To prepare for it, Israel has invested millions of dollars in strengthening its northern border fence and the Army patrol road that runs just inside it. At some points, a triple row of barbed-wire fence, from which electronic sensors jut out every few feet, guards Israel's frontier.

At other key and particularly vulnerable points, such as the Lebanese farm and grazing land just north of the Israeli border town of Metulla, the Army has dug a 10-foot-deep ditch designed as a barrier to suicide car-bomb drivers. It has been dubbed "the good trench," a parody of the nearby so-called "good fence" that for years has served as an access point to Israel for the Lebanese Christians of nearby villages.

The Israelis are issuing special identity cards to the more than 100,000 Lebanese who live in the security zone to control access to the territory. Earlier this week, they also blew up the homes of three Lebanese families they described as sympathizers with guerrillas who have infiltrated the security zone. This month, U.N. officials in the region said, guerrillas launched more than 40 attacks on the remaining Israeli forces and their local Lebanese allies.

A few miles east of Metulla near the Lebanese village of Majidiye, where the borders of Lebanon, Syria and Israel converge, more last-minute preparations were going on today. In a meadow already turned various shades of yellow and gold by the spring sun, a World War II-vintage Sherman tank, black smoke belching from its diesel engine, lumbered down a rocky slope while it sprayed a nearby hillside with a machine gun.

From the roof of a dilapidated cement building, an Israeli Army major watched as the tank's gunfire started a grass fire on the hillside.

"I think they are quite good, better than I expected," the major said of the tank crew. The major, who under Israeli Army regulations cannot be identified by name, is training members of the South Lebanon Army on how to handle its 40 Israeli-supplied tanks, mostly Shermans and a few equally antiquated Soviet T54s.

The hour of the South Lebanon Army, Israel's new surrogate force in Lebanon, is about to arrive. It will not be left entirely on its own, for the Israelis have said they will continue to patrol the security zone with their own forces and, when necessary, man observation posts.

In an interview today with western reporters at his headquarters in the Lebanese Christian town of Marjayoun, the commander of the South Lebanon Army, Brig. Gen. Antoine Lahad, refused to discuss his force's size, composition or financing. He said his army's size was "adequate" and that his soldiers represented all the ethnic groups of southern Lebanon: Christians, Shiite Moslems and Druze.

Israeli officials said the South Lebanon Army, including its affiliated local village guards, numbers between 1,200 and 1,500 men and that it is made up overwhelmingly of Christians. They said it is totally dependent on Israel for logistical and financial support.

Lahad, a Christian, repeated what has been his message for several months: that the South Lebanon Army can control the security zone against local threats, but that if Syrian-backed outside forces attempted to intervene, it will need direct Israeli countermeasures to support it.

Israel's new security zone is considerably larger than the enclave that it established and controlled following its 1978 invasion of southern Lebanon through the militia of the late Lebanese major Saad Haddad. It is about six miles wide along the Lebanese coast, somewhat narrower along some stretches of the central border and reaches its greatest depth of 15 miles here at Kfar Houne. It runs this far north, according to Israeli officers, because Lahad wanted an access point to continue his support of the Christian town of Jezzin, located about six miles beyond the security zone.

Within its confines, South Lebanon Army soldiers bump up against the soldiers of the U.N. Interim Force in Lebanon (UNIFIL), and, as in the days of the Haddad militia, there have already been incidents. Earlier this week, Israeli soldiers had to intervene in a confrontation between the two forces that reportedly involved shooting into the air and the ramming of several vehicles.

Lahad blamed this on a misunderstanding and UNIFIL's refusal to recognize his militia as a legitimate security force in southern Lebanon. He made it clear that he will continue to challenge the U.N. force's attempts to impede his troops' movement through the area and that he expects Israel to back him up