In the battered main square of this southern Lebanese town, which had been under virtual Palestinian siege for almost two years, Christian Lebanese waved and shouted "Shalom" yesterday as Israeli tanks and trucks roared through the street on their way north.

A Christian Lebanese school teacher, Francis Rizk, had nothing but praise for his allies, the Israeli army.

"We have lived like rabbits under the ground" because of fear of the Palestinians, said Rizk, who guided a group of foreign correspondents through Marjayoun. He told of Palestinians killing Christians and said, "President Carter talks of human rights, but we are humans, too."

Over the last two years, Israelis and Lebanese Christians in southern Lebanon have become allies against the Palestinians. The area had long since passed out of the effective control of the central Lebanese government, and Israel - by opening its border to the Christians - has forged a strong working alliance with them.

To dramatize the closeness between the Lebanese Christians and Israel, Rizk told how Israeli donors had given their blood to save Christian wounded. Now, he said, the blood of Israelis flows in the veins of Christians, and they are as one.

Marjayoun, five miles north of the Israeli border and within the six-mile-deep security belt Israeli forces seized this week, gave all the appearances of a liberated town yesterday.

People were smiling as they came out of the buildings, whose red-tile roofs were holed by shells and whose walls were spattered by rocket and artillery rounds.

Across the valley was Khiam, until Wednesday a Palestinian stronghold but now, we are told, a ghost town whose inhabitants either fled or were killed.

To the east, under the brooding, snow-capped shoulder of Mount Hermon at the Syrian border, we could see what was left of the Palestinian stronghold that came to be known as "Fatahland." There, the last pockets of resistance within the new Israeli security belt reportedly were being cleaned out.

To the west, looking down from the ridge liine, we could see the silver thread of the Litani River and the outline of the 12th century Beaufort Castle which, lying across the Litani River, is off bounds to Israeli troops. But it is from there, and the town just below, Arnoun, that Palestinian gunners continued to harass both the Christian villages in Lebanon and towns in northern Israel as Israeli guns and planes attempt to quiet them.

As we milled about Marjayoun's main square, interviewing merchants who were opening the bullet-riddled shutters of their shops, the sharp crack of a rocket sounded a few streets away.

A Christian militaman, speaking Hebrew that he had acquired during his close association with the Israelis, said nervously that the next round would land among us in the square. But the townspeople seemed unconcerned as they crowded around to be photographed and interviewed.

It was not long before the Israeli answer came in the form of fighter bombers diving down on Arnoun, as they had earlier in the day bombed Beaufort Castle, and great clouds of smoke rose from the Palestinian position.

It was only a year ago that I had visited Arnoun from the north with the Palestinians, and heard their side of the struggle against then covert Israeli involvement in southern Lebanon.

Palestinian-held towns south of the Litani River, such as Bint Jbail looked as shell pocked and battered as Marjayoun, but now the Israeli army is in the town.

Our group came to town a day after the Israeli army, as the first foreign correspondents to visit Israel's newest occupied territory.

That all was not as it should be in northern Israel became clear as our car came under rocket attack before we even reached the Lebanese frontier.

As we approached Israel's northern-most town, Metullah, the deceptively innocent pop and plume of smoke from a 122mm rocket that hit a hundred yards from the road showed clearly that Israel's northern border communities are still vulnerable to attack. Moments later, a second landed closer.

The alarm siren was sounding as we arrived in Metullah for a rendezvous with a press bus and the whistle and crack of another rocket sent the correspondents diving for cover.

We learned that the road from the Israeli border to the Christian-held towns of Qlaiaa and Maryjayoun had been under shell and rocket attack in the night and during the morning.

It was not until afternoon that the Israeli defense force decided it was safe enough to let us proceed at last, with our nervous Israeli escort officers.

We were packed into a blue Israeli bus and allowed to proceed into Lebanon.

As we crossed the border into Lebanon, we could see Israeli armored personnel carriers by the side of the road, their tracks blown off by land mines.

scattered artillery fire could be heard throughout the day. Israeli armor continued to move into Lebanon and often the sky was full of sounds of Israeli helicopters and planes. But there was none of the machine gun and small arms fire that indicates fullscale ground combat - at least not in this area.

The leader of the Lebanese Christian forces, Maj. Saad Haddad, had told us that the Palestinians seemed to be directing more of their fire at Christians towns than at the Israeli army. But he said his people were going to try to resume a normal life after two years of living in shelters and foregoing school for their children.

Visiting with reporter later yesterday in Metullah, Haddad said, "Now I feel much better. Israel has dealt with the Palestinians."

One does not know how long the honeymoon between the Christians and the Israelis will last now that their common enemy, the Palestinians, have been driven back farther into Lebanon.

Nor is it possible to say how long the Israeli army will remain here. Prime Minister Menachem Begin has said they will remain until there is an agreement providing that the Palestinians will not return.

Our group jokes with Israelis that 11 years ago their older brothers had said that Israel would give back the West Bank of the Jordan River just as soon as there was a political settlement.

"And we meant it," one of the Israelis said.

But for all the power and might of the Israeli force that was on display here today it is difficult to see how this invasion can free Israel from terrorist attacks such as the one in which 36 Israelis were killed on the Tel Aviv highway last weekend.

The perpetrators of this act, an Israeli military spokesman has said, were trained and launched from bases, far to the north, near Beirut.

Israeli occupied territory grows ever bigger, but Israeli's security seems enhanced only in the narrow tactical sense which the political and startegic problems grow even bigger.