Palestinian guerrilla leader Yasser Arafat struck out at the United Nations peace-keeping force serving in southern Lebanon today, accusing it of facilitating the Israeli invasion and of "collaborating with the Israeli troops."

In a statement carried here by the Palestinian news agency, Wafa, the Palestine Liberation Organization chief accused the U.N. Interim Force in Lebanon (UNIFIL) of allowing the invading Israeli Army to use its bases and roads and said it "helped them stab the Palestinians in the back."

The charges against the 7,000-man U.N. contingent stationed in southern Lebanon to prevent Israeli-Palestinian clashes brought out into the open the anger raging among Lebanese officials and Palestinians over its total failure to do anything to stop the Israeli invasion.

The weakness of the U.N. peace-keeping mission here seems likely to have enormous repercussions on its future role in Lebanon in any future negotiations for a new buffer zone under U.N. auspices there after the fighting is over.

The Israelis have been suggesting that a bigger and stronger UNIFIL force might be one solution to solving the problem of creating a deeper, 25-mile demilitarized strip to protect northern Israel from future Palestinian shelling.

It is far from clear, however, whether the Lebanese or Palestinians would trust UNIFIL to shield them from the Israelis.

Arafat was quoted today as having told one U.N. official here that "the least you could have done was to shoot over the heads the way you do with us."

The only slightly effective resistance UNIFIL forces were reported to have put up came from a contingent of Nepalese troops that blocked the Israeli advance on the Khardali Bridge across the Litani River just below the Crusader-era Beaufort Castle, forcing the Israelis to put up pontoon bridges to go around them.

There was also one soldier killed from a Norwegian contingent that briefly resisted the Israeli advance along a road below Mount Hermon.

The ineffective performance of the four-year-old UNIFIL force has been a source of enormous embarrassment to U.N. officials here.

Today Gen. William Callaghan, the Irish commander of UNIFIL, made his first formal protest by letter to the Israelis, noting that he had only been informed about the invasion at a meeting with the commander of Israel's northern forces, exactly 28 minutes before it began at 11 a.m. local time Sunday.

"You will recall that I strongly objected to your statement and protested against any such wrong and unacceptable course of action," Callaghan's letter said, according to U.N. sources here.

In the letter, the UNIFIL commander said Israeli forces "in an endless stream continued to pass through and occupy numerous UNIFIL positions in utter disregard of the basic function of the force as laid down in the mandate it received from the U.N. Security Council."

He said this had been done despite "numerous protests" about the "unacceptable behavior of your forces," which the UNIFIL commander accused of "massively violating and illegitimately occupying" the area held by U.N. troops.

It was believed to be the first formal UNIFIL protest to the Israelis over their invasion.

It is probably too soon to judge whether UNIFIL is now finished in Lebanon, but Western diplomats and analysts here said they doubted that it has much of a future.

"If Israel does not withdraw immediately or does not accept a U.N. Security Council request to do so fast, then you can write off UNIFIL and also any hopes of expanding it northward," a Western diplomat said.

He questioned whether any participating countries would add reinforcements to their contingents or whether others would offer to join an expanded U.N. force.

"If 7,000 soldiers are not enough to do the job in the present UNIFIL area, then how many more would you need in an area between the Litani and the Zahrani rivers, which is bigger and more difficult terrain than that?" a military analyst remarked, referring to the land that would have to be added to create the 25-mile demilitarized zone the Israelis are talking about.