Israeli military forces took control of Lebanon's entire southern frontier region yesterday and Israel said it would hold the territory until an international agreement is reached barring the presence of Palestinian commandos.

In a swift invasion that took less than 18 hours, Israel poured armor and thousands of troops into southern Lebanon and bombed and strafed Lebanese towns and Palestinian camps along the coast as far north as Beirut's southern suburbs.

After occupying a six-mile-deep swath of Lebanon, reaching almost 60 miles, from the Mediterranean Sea to the foothills of Mount Hermon, Israel said its troops had halted their advance, adding that they had been successful in clearing the area of "possible terrorist concentrations."

There were no firm casualty reports from either site. Israeli officials said a hundred or more Palestinians had been killed but they released no information on Israeli casualties. Diplomatic sources in Beirut said several hundred Palestinian guerrillas and civilians had been killed.

The United States reacted cautiously to the invasion yesterday and sought to discourage any sense of crisis.

Arab countries, on the other hand, called on the United Nations and the major powers to intervene and Egypt, in an especially harsh statement, condemned Israel's "hasty resort to aggressive operations" as an effort to "systematically annihilate the Palestinian people."

While Israeli Prime Minister Menachem Begin said he saw no reason why the invasion should affect peace talks between his country and Egypt, Egyptian Foreign Minister Mohammed Kamel said it "represents a further obstacle on the road to a just, permanent and comprehensive peace in the Middle East."

As Israeli troops tightened their hold on the territory they had seized last night, there was no indication that the operation would expand into a confrontation with the 30,000 Syrian troops present in Lebanon as a peacekeeping force.

Damascus announced that it had agreed to a request from Lebanon to provide air defense against further Israeli raids, but there were no moves by Syrian forces to carry out this agreement. Syrian forces remain several miles north of the area occupied by Israel.

There were scattered reports of ground fighting between Israeli and Palestinian forces but it appeared that most guerrillas in the border area had moved to the north before Israel launched its invasion Tuesday night.

Israeli troops reported overran at least five villages in southern Lebanon that had been Palestinian strongholds and, according to unofficial reports, took several prisoners.

In some towns, Israeli Defense Minister Ezer Weizman said, fighting had been heavy. "It wasn't a walkover," he told reporters.

The invasion came three days after a Palestinian commando squad landed on the Israeli coast and commandeered a tourist bus in a raid that ended with the death of 36 Israelis and nine of the 11 commandos. The other two commandos were captured.

Weizman and Begin said yesterday that the purpose of the invasion unlike that of previous retaliatory raids, was not merely reprisal but a broader effort to clear the area north of the Israeli-Lebanese border once and for all of Palestinian commandos.

Lt. Gen. Mordechai Gur, Israel's chief of staff, told reporters that Israel intended to establish a five-mile-deep cordon sanitaire along the border that would link the Lebanese Christian villages there into buffer zone that would provide security to Israel.

Christians in southern Lebanon, where the central Lebanese government no longer exercises authority, have cooperated with Israel in return for military support against Palestinians and Lebanese Moslems.

Weizman said Israeli troops would occupy the area they seized it only until security agreements are concluded. He said Israel has no territorial claims on the land, which it considers "part of sovereign Lebanon."