Hundreds of tanks and armored vehicles pushed across the Lebanese border here yesterday, passed U.N. peace-keeping forces that have been in place since the 1978 Israeli invasion of Lebanon and moved northward in a massive invasion with the declared objective of putting Palestinian gunners out of range of Israel's northern frontier.

The hills of southern Lebanon echoed with almost continuous artillery fire throughout the day. Israeli guns in the enclave controlled by Israeli-supported Christian militias led by Maj. Saad Haddad pounded Palestine Liberation Organization positions around Nabatiyah and the PLO-held stronghold in Beaufort Castle, a Crusader-era ruin overlooking the Litani River. This morning, the Israeli Army command announced that an infantry force captured the strategic castle as its operation to "clear out terrorist concentrations" in southern Lebanon continued throughout the night.

Palestinian gunners returned the artillery fire toward the edge of Metulla, where armored columns assembled, and on civilian settlements in northern Galilee.

Heavy smoke billowed from Beaufort as Israeli jets repeatedly bombed the artillery observation post and nearby gun emplacements.

Near the castle, shortly before 8 a.m., an Israeli Skyhawk fighter was hit by antiaircraft ground fire, with part of the plane enveloped in bright orange flame before the pilot bailed out and fell into PLO-controlled territory, according to eyewitnesses. He was taken into custody and later presented at a PLO press conference in Beirut, where he said he was being treated well.

In a communique issued several hours after the morning invasion began, the Israeli Cabinet said it had taken the action "to place all civilian population of the Galilee beyond the range of terrorist fire from Lebanon where their bases and headquarters are concentrated."

No further explanation was given for the decision to invade, which climaxed three days of escalating tension sparked by the attempted assassination Thursday of the Israeli ambassador to London. Israeli warplanes bombed Beirut on Friday and returned to Lebanon Saturday for further attacks as Israeli and Palestinian forces exchanged artillery fire across the Lebanese border.

An Israeli cabinet spokesman said the Army had been instructed to drive back the guerrillas to at least 40 kilometers 25 miles from the Israeli-Lebanese border. Judging from accounts by officials of the U.N. force in Lebanon, the Army's tactical objective appeared to be twofold: to push Palestinian guerrillas north at least as far as the Zahrani River, creating a new cordon sanitaire beyond the frontier stretching at least as far as the 18- to 25-mile range of the 130-mm artillery that the PLO has been using to shell settlements in Galilee in northeastern Israel; and to destroy the Palestinians' military facilities in southern Lebanon.

Israel has maintained that about 6,000 Palestinian guerrillas are in Lebanon, with 1,500 of them positioned in a narrow pocket around Tyre on the Mediterranean Coast. The government has said that since Israel and the PLO agreed to a U.S.-arranged cease-fire last July 24, the PLO has deployed 20,000 tons of military equipment in southern Lebanon, including heavy artillery, Katyusha rocket launchers capable of firing 40 rockets in one salvo, and about 80 tanks, including Soviet-built T55 models.

It is this equipment, along with training bases and PLO headquarters, that Defense Minister Ariel Sharon has repeatedly declared must be destroyed if Israel's northern border is to be secured. Although it was widely assumed that, despite the inclination of Sharon and Begin to move against the PLO strongholds, U.S. disapproval and Begin's scheduled visit to the United States this month dictated restraint. The London shooting and clashes that followed, however, apparently turned the balance.

Israel's three-pronged invasion seemed intent on closing escape routes the guerrillas might use, thus avoiding the situation that occurred in the March 1978 invasion when guerrillas fled north across the Litani River from the advancing Israeli forces and then infiltrated back into the border region after the U.N. troops had been deployed and the Israeli Army had withdrawn.

In 1978 the Israelis created a buffer zone along the border south of the U.N.-controlled areas and turned it over to the Israeli-backed Christian militias Haddad leads.

Shortly after daybreak, long lines of tanks and armored personnel carriers queued up on a dirt road leading along the border chain-link fence just west of this town and then rolled through a 15-foot-wide gap in the fence into the enclave controlled by Haddad.

As they entered the enclave, PLO rockets exploded about 50 yards from an observation platform that normally is crowded with tourists visiting the "good fence" entry through which Lebanese residents of the enclave pass on their way to jobs in Israel.

The steep, winding road leading from Tiberias south of here on the Sea of Galilee was jammed with Army vehicles throughout the morning, all of them headed north in a swift Israeli Army mobilization of reservists and Army troops. Other troop and armored units gathered at staging areas south of Metulla, apparently preparing to be called as reinforcements.

However, by midafternoon, the massive Army presence in Metulla had vanished beyond the hills of the Christian enclave, and the town took on an eerily quiet atmosphere. Residents emerged from underground bomb shelters and resumed their normal activity or gathered on hilltops to watch jet fighter-bombers streaming overhead toward the north. Some gathered on rooftops of houses to watch the artillery duel that continued for most of the day and into the night.

"I hope this time they do what needs to be done and put an end to the PLO for good," one resident said as he stood in front of his shingled bungalow and watched the last of the armored vehicles move across the border.

The road north to the border was dotted with Israeli soldiers, in full combat dress, hitchhiking to the front. More than 1,000 public buses from Tel Aviv and Jerusalem were mobilized to transport reservists north in what was reported to be a "war call-up" as extensive as during the 1978 Litani operation.

In Qirat Shemona, the streets were deserted, except for a few residents who stood on the sidewalks and watched the Army convoys pass. The town's only hotel was hit by a Katyusha rocket Saturday night, and most residents have remained in bomb shelters.

From the roofs of buildings here, lines of supply trucks and troop carriers could be seen headed toward Marjiyoun in Lebanon, on a new road constructed by the Israelis from the Golan Heights border.

Following the Cabinet session, which began hours after the first armored columns entered Lebanon, Cabinet Secretary Dan Merridor said Prime Minister Menachem Begin and his ministers had ordered the Army to "place all the civilian population of the Galilee beyond the range of the terrorists' fire from Lebanon, where they, their bases and their headquarters are located."

The Cabinet communique gave no projected duration of the invasion, which it called "Operation Peace for Galilee," but it said, "Israel continues to aspire to the signing of a peace treaty with independent Lebanon, its territorial integrity preserved."

Although the reference to peace with "independent" Lebanon, and its implication of a necessity for an end to the presence of 23,000 Syrian troops there, appeared to suggest prolonged Israeli control of the territory, Israeli officials tonight said there are limited objectives to the invasion and that the hope is to withdraw soon.

"We have no aspirations for a single inch of Lebanese territory," Foreign Ministry Director General David Kimche said on Israeli radio. "Our sole aim is to free ourselves from the threat of terrorism."

Kimche said Israel had shown "remarkable restraint" in the face of repeated PLO cease-fire violations during the past 10 months but that the attempted assassination in London of Israeli Ambassador Shlomo Argov was "the last straw."

"We feel completely justified in defending the lives of our citizens. We are not the aggressors. We are acting in complete and total self-defense of the lives of our citizens," Kimche said.

Foreign Minister Yitzhak Shamir, in a dinner speech tonight, said, "Our war against the PLO is a struggle for the sake of peace, because the PLO has become a major obstacle on the inevitable march toward the goal of peace."

The invasion immediately won the support of the opposition Labor Party, with both party leader Shimon Peres and former prime minister Yitzhak Rabin urging Israelis to rally behind the government decision.

"There is no choice but to ensure normal life for the inhabitants of northern Israel so that they should not be subjected to the whims of this and that terrorist leader or the threats posed by terrorist organizations. There was complete agreement among all the Zionist political leaders," Rabin declared.