Prime Minister Menachem Begin and military leaders here said yesterday that the purpose of Israel's invasion of southern Lebanon was to force a long-sought political agreement that would keep Palestinians commandos out of the sensitive Israeli border region.

This broad political goal gives the present Israeli invasion far greater significance than previous raids and air attacks, in which the chief purpose was to retaliate for Palestinian operations against Israel.

"This is not a reprisal operation in the usual sense," Defense Minister Ezer Weizman said at a press conference "The operation comes - and I believe it will be successful - to destroy and uproot as far as possible terrorist concentrations in southern Lebanon."

To underscore Israel's control of southern Lebanon, Begin and Weizman visited the area yesterday, greeting Israeli troops near the front lines.

Begin, at a press conference, said that the Israeli invasion should not affect negotiations with Egypt toward a Middle East peace and he said those talks could go on if Egypt wanted them to.

Israeli officials said the military operation was completed yesterday when Israeli forces took full control of a six-mile-deep swath along the entire 60-mile Israeli-Lebanese border.

Israeli Chief-of-Staff Lt. Gen. Mordechai Gur said the air, artillery and ground strikes had killed about 100 Palestinians and Weizman said it was possible that more had been killed by the bombing. There was no announcement here of Israeli casualties.

Diplomatic and Palestinian sources in Beirut said the death toll was in the hundreds, including guerrillas as well as civilians. Palestinians said they killed or wounded about 300 Israeli troops.

"Our army was not sent into southern Lebanon for the purpose of staying here," Begin said.

"We want an agreement and an arrangement as a result of which in all those places from which the murderers were ejected, they should not return," he said, "and in general that southern Lebanon still not serve as a base for further attacks against the state of Israel and its citizens."

"This is the real purpose of the decision taken by the government of Israel and the operation carried out valiaently," Begin said.

The prime minister said that all the concerned parties, with the exception of the Palestine Liberation Organization, would be consulted. It was clear that Begin intends that Israel will hold southern Lebanon until such an agreement is reached.

Israel has always felt that Syria could stop Palestinian activity in southern Lebanon because the Syrians control the routes of supply and entry to the region.

The Palestinian raid last Saturday, in which 36 Israelis died, has provided the opportunity for Israel to put its long-held plan into effect. It provides for a cordon sanitaire, which many Israeli planners have long considered to be a strategic necessity and which Israel had tried with limited success, to maintain by helping Christian militias against the Palestinians in the area.

The main battles are all but over and the Israeli forces are now moping up the last Palestinian strong points. But few knowledgeable Israelis believe that the Israeli presence in southern Lebanon will end quickly and the political battle over the conditions of an Israeli withdrawal could take months.

Weizman said that it was not Israel's aim to "hold on to or remain in southern Lebanon." He said that "nobody sees south Lebanon as part of Israel - never did and never will."

But he made it clear that Israel intended to clear the area and to prevent it from again becoming a base for Palestinian operations against Israel. He said that Israel would control the area "as long as we find it necessary and as long as nobody else puts their foot down" to prevent the area from being used as a jumping off place for attacks on Israel.

"Therefore we don't see this as one operation but as a continued preventive action on our northern border," he said.

The ideal situation would be, Weizman said, if the "legal government of Lebanon could control southern Lebanon."

He mentioned that Syria and Jordan, although not friendly toward Israel, did not permit their territory to be used as a base for terrorist attacks. But Lebanon's President Elias Sarkis has said he has no control over southern Lebanon.

The clear indication was that Israel would remain in the border given until someone else was in control and in a position to stop hostile activity by Palestinians.

As for the Christian forces in southern Lebanon, whose hard, pressed positions have now been relieved, Weizman said that they had "their part in what has to be done in a place which is, in the end, their own country, their settlements, their villages, their land."

The former Israeli chief of military intelligence Aharon Yar IV, now head of Tel Aviv University's Institute of Strategic Studies, said on Israel Radio that the plan that the government put into action yesterday morning had been ready for some time. The military phase would end quickly, he said, but the political phase would take longer and it would be up to the government to make clear that the price for its withdrawal would be security on the northern border.

Yigal Allon, former foreign minister and now an opposition member of parliament, said that the government had taken the "right measure at the right time." Israel, he said, should insist on a political settlement on the future of southern Lebanon or at least retain the option of preventing the return of Palestinian forces to the region.

Israel has tried to signal the Syrians, who make up the bulk of the 30,000-man peacekeeping force in Lebanon, that the operation was limited to south of the Litani River, an area that has always been off bounds to the Syrians at Israel's insistance. "I do hope the Syrians will understand that this is a limited action in south Lebanon," Weizman said. "I do hope the government of Lebanon will understand that this is a preventive action to protect the lives of Israelis."

Gur told reporters that "in this operation the intention is to take control mainly of the region of the strip . . . between four and six miles deep. In the course of the operation we are also hitting more distant targets but we are not entering them with land forces."

The Israelis have been hitting targets beyond the limited area in which the ground troops are operating. According to a military spokesman, Israeli planes bombed the coastal port of Tyre as well as Damour and Mazh Uzaiye, which are only a few miles south of Beirut.

"But we have no intention of reaching such distances on the ground," Gur said. According to the Israelis, the Palestinians who struck at last weekend were trained in and departed from Damour.

Israeli planes came under fire from Syrian antiaircraft guns, the spokesman said, but did not return the Syrian fire and returned to base safely.

Gur said that by midmorning most of the main ground objectives had already been taken in the narrow strip from the Mediterranean to the foothills of Mount Hermon.

The principal Palestinian camps that the Israelis have captured are, according to Gur, Ein Nakura near the coast; Bint Jball, Yarun and Maroun al Ras in the center of the country; Taibe and Khaim and the Palestinian camp of Ibl as Saqi to the east, in the high country known as "Fatahland."

Resistance was heavy in some of the Palestinian strong points but light in others, Gur said.

Some Israeli towns and settlements came under sporadic rocket and artillery fire during the night and morning, he said, but there was no announcement of Israeli casualties.