Humiliated, weak, despised by many, possibly error-prone and losing credibility, the United Nations peacekeeping force in Lebanon is what the diplomats like to call a delicate mechanism of crisis control.

In practice, that means that the 6,000-man force, made up of troops and support units from 12 nations, operates as human tripwires, refereeing in the best Marquis of Queensbury tradition a no-holds-barred donnybrook pitting the Palestinian guerrilas and their Lebanese ruffian friends against Israel and its Lebanese ruffian allies.

Despite the stiff-upper-lip tradition affected around this seaside headquarters -- which twice has been attacked at point-blank range and heavily damaged by belligerents in the crazy-quilt war in southern Lebanon -- few in the U.N. force believe the official policy proclaiming that its real strength lies with world public opinion.

The world, in fact, starting with the United States, has conveniently forgotten about UNIFIL -- as the force is known, after the initials of its formal name, U.N. Interim Force in Lebanon. Washington appears to have abandoned serious interest in what actually was a U.S. brainchild, conceived as a polite means of forcing Israel to withdraw from southern Lebanon after it invaded in March 1978.

Yet the U.N. force is kept busy as attempted Palestinian and leftist infiltrations, which had sunk from a monthly average of 30 to three or four in late 1979, have crept back since last fall to the 12 to 15 range.

Because of the hostile atmosphere in southern Lebanon and the restrictions on military response by the U.N. troops, it seems miraculous that they have suffered no more than 58 fatalities -- including two Monday -- and only 23 of these by hostile fire.

The peacekeeping force now seems to be a genuine source of interest only to the worried governments of the countries providing infantry troops: Fiji, Ghana, Ireland, the Neatherlands, Nigeria, Norway and Senegal. Additionally France, Italy, Norway and Sweden provide logistic units and 10 Nepalese serve at headquarters.

Now all sides are watching as a new U.N. commander, Maj. Gen. William Callaghan, is preparing to have UNIFIL serve notice on all the belligerents that it means business and that accommodation is to be frowned on.

U.N. troops are expected to be authorized to shoot back in self defense. Night patrols are being stepped up. Electronic sensors may be used. Already U.N. forces have begun using roving checkpoints -- instead of the easy-to-skirt static positions -- with remarkably improved results in curbing infiltrations. Callaghan is also considering establishing a multinational reaction fast force to hit back quickly at marausing parties from any of the belligerents.

Almost from the very start of its peacekeeping activity in Lebanon, things have gone downhill for UNIFIL.

Palestinian guerilla leader Yasser Arafat agreed to cooperate with the U.N. force, but only after his men humiliated a French battalion that tried to take control of the area around the Mediterranean port of Tyre. Arafat argued that since Israel had not occupied that territory, the U.N. force had no right to move in From then on, UNIFIL made ever greater concessions to the guerrillas.

Despite the U.N. vote giving the peacekeeping force the mandate to take control of the entire area occupied by Israel in its invasion, the Israelis handed over only two-thirds of the occupied territory south of the Litani River, entrusting the rest of the area nearest the border to their Lebanese Christian ally, Maj. Saad Haddad.

Ever since, the Lebanese central government has been trying to persuade the United Nations, the United States and anyone else who will listen to honor Security Council resolution 425.

Adopted March 19, 1978, the resolution instructs the U.N. force to "confirm the withdrawal of Israeli forces, rstore international peace and security and assist the government of Lebanon in ensuring the return of its effective authority in the area."

The Beirut government argues that UNIFIL's failure has complicated its already formidable problem of solving Lebanon's internal political problems.

Many U.N. officials said at the time that UNIFIL should refuse to participate, at least until wandering international attention focused more effectively on the situation. But now, the U.N. force, under its mandate, must stay until Beirut asks it to leave.

If it left, virtually everyone from Callaghan on down predicts Israel would fill the vacuum and some fear the Israelis would keep going north.

Unable to move to the Israeli border the peacekeeping force instead tries to "consolidate the ground we hold." That is no easy task, with both armed camps determined to nibble away the six-mile-wide U.N. turf so they can get at each other.

A sense of futility has built up among U.N. soldiers faced with the belligerents' refusal to accept the spirit or letter of the mandate and their tactical advantages. Unlike the peace-keeping forces, the Palestinians and the Israeli-backed Christians have an intimate knowledge of the terrain, good intelligence operations and the habit of striking at night in blitzkrieg fashion.

"Look at the way the Israelis and Palestinians treat us," a Norwegian officer complained.

More than 200 U.N. vehicles have vanished since 1976, most of them hijacked by "armed elements," as the Palestinians and their allies are called, and most of those that are found have been stripped of their radios.

"De facto forces," the U.N. force's name for Israeli-backed militiamen under former Lebanese Army Maj. Saad Haddad, harass U.N. observation posts and restrict their traffic through their enclave to a couple of times a week.

UNIFIL's rules of engagement require a challenge and then a warning shot before a soldier may fire for effect, and then without intent to kill. Both sides have taken advantage of this directed tameness to humiliate U.N. soldiers and officers by hijacking vehicles and forcing them to return to their units on foot, sometimes without shoes and shirts.

"No one wants to take on an Israeli patrol becuse no one wants casualties on either side," a U.N. officer said, "but I am convinced if we were tougher we's get more respect. The Israelis aren't that good. It's just that the others are appaling."

A particularly difficult task faces the Dutch. Fijians and Irish in policing the "Iron Triangle," an area where 250 to 300 Palestinian guerillas try regularly to escape from their tolerated positions to move south against Haddad and the Israelis.

The PLO is allowed to resupply the triangle denizens, who are supposed to number no more than 190, but only with food, water and medicine, according to an agreement between UNIFIL and Arafat.

Their presence is justified by PLO insistence -- contested by Israel -- that it was laid down in the 1969 Cairo accords whereby Lebanon guaranteed the PLO 14 positions near the Israeli border.

One irate U.N. officer vented his frustration recently with Israeli charges that UNIFIL was not stopping infiltrators. "We're stopping 95 percent of them," he said, "and we'd do even better if the Israelis would get Haddad off our backs so we could devote our full time to infiltration problems."

The Norwegian forces have a similar problem with something called "the nest" that ties down two platoons who keep 24-hour-a-day watch over 12 Palestinian and Lebanese leftist guerillas. Every five days the guerrillas are escorted back to their lines and replacements take their place in the nest. Weapons are left in the nest.

With Israeli backing, Haddad's forces have taken over four portions of U.N. territory and Palestiniand and their Lebanese allies have expanded the Iron Triangle.

Almost always, once land is taken, the perpetrators tend to get away with it. "There's a lot of weeping and conferences," a diplomat said, "but you almost never get it back."

After some ups and downs in their attitude, the Palistinian guerrillas since October have become much more aggressive; the helicopter of Maj. Gen. Emmanuel ERSKINE, THE ghanaian U.N. commander until last month, has been hit twice by their fire.

The most effective U.N. battalion facing the Palestinians has been the Fijans, who have stood their ground, suffering casualties that other nationalities would find difficult to justify to their governments.

Complicating the UNIFIL task is the failure of Arafat's Fatah, the parent Palestinian organization, to maintain discipline withing the ranks of its often more radical fellow combatants. In a mirror image of Israel's relationship with Haddad, the PLO has found it useful to blame much of the turbulence it causes on its Lebanese allies who enjoy full rights as citizens of the territory the U.N. forces operate in.

Although U.N. troops regard both belligerents with a healthy, if negative neutrality, the PLO feels it is getting the better of the deal.

"Whatever our reservations about UNIFIL from a military viewpoint," a PLO liaison officer in Tyre, said, "We're happy because every year 12,000 UNIFIL troops rotate through here and are so angry at the Israelis that they become our ambassadors back home."

However overdrawn that claim may be, Dutch, Irish and Norwegian diplomats say privately that the presence of their troops has eroded pro-Israeli public opinion at home.

Despite the increase of Palestinian-initiated incidents, the U.N. officers note that Israel no longer is demanding dismantling of the peacekeeping force.

At the same time, Lebanese villagers go along with the peacekeeping force because they want protection to go about their daily lives. They feel that even though there is still danger, they are still better protected than Lebanese living in "Haddadland" or Palestinian-controlled areas to the north.

The Lebanese government and local villagers would like to turn UNIFIL into a fighting force, but know this is unlikely. "UNIFIL are only witnesses to our drama," a dispirited civil servant said in Beirut, "and we don't need any more witnesses."