Israeli forces, supported by heavy air and naval bombardments, began a massive ground assault against Palestinian guerrillas in southern Lebanon yesterday. Syria said its troops had joined Palestinians in opposing the Israeli attack, raising the specter of another full-scale Middle East war.

The Israeli action represented a total breakdown of last year's cease-fire in southern Lebanon and brought the worst fighting in the region since March 1978, when Israel mounted a similar invasion of Lebanon.

An appeal by President Reagan to Prime Minister Menachem Begin to "do what you can to avoid military steps that could lead to a widening of the conflict" went unheeded. Reagan dispatched special envoy Philip Habib to the region to try to restore peace.

The United Nations Security Council adopted a resolution calling for an immediate Israeli withdrawal. Details on Page A10.

Thousands of Israeli soldiers accompanied by tanks and armored personnel carriers moved across the 60-mile border, pushing through the narrow strip controlled by the Lebanese Christian forces allied with Israel and the zone occupied by U.N. peace-keeping units.

Other Israeli troops, ferried in by boats and helicopters, landed along the coast, some as far north as the Zahrani River, in an apparent attempt to block a Palestinian retreat to the north.

Israeli warplanes swarmed over Lebanese sky, striking at suspected Palestinian positions, and Israeli ships shelled targets along the coast.

Israel long has warned that it would move against what it said was a large-scale buildup of Palestinian forces in the area. The Palestinians have said they were bolstering their units in anticipation of an Israeli strike.

The Israeli action was similar in size to its 1978 invasion. Then, however, the operation was confined to the territory south of the Litani River, and that enabled the Palestinian guerrillas to escape north across the river.

According to Washington Post correspondent William Claiborne, the apparent objective of the current invasion is to drive the guerrillas north to the Zahrani River and to clear an 18- to 25-mile wide zone that will put Israeli settlements in Galilee out of the range of the 130-mm artillery used by the Palestinians.

U.N. officials said that after several hours of fighting the Israelis were closing in on the guerrilla-controlled port of Tyre with 100 tanks and 100 personnel carriers.

Another column was said to be driving on Nabatiyah, north of the Litani. A third major thrust appeared to have been launched through the Golan Heights and moving to the northwest near the Lebanese village of Chebaa.

A Norwegian member of the U.N. force reportedly was killed during the fighting.

The assassination attempt against the Israeli ambassador to Britain Thursday night sparked the current round of violence in the area. Israel retaliated Friday with air strikes into Lebanon, and the Palestinians countered with artillery fire into northern Israel.

Israel announced that one of its attacking planes had been shot down, the first time in at least seven years that Israel has lost an aircraft in combat. The Palestine Liberation Organization produced the pilot at a news conference in Beirut.

Palestinian spokesmen said last night that PLO forces had inflicted "more than 200 casualties" on the Israelis.

In a letter to Reagan, Begin said the Israeli "Army has been instructed to push back the terrorists to a distance of 40 kilometers 25 miles to the north so that all our civilians in the region of Galilee will be set free of the permanent threat to their lives. We do not covet one inch of Lebanese territory." In Jerusalem, a Cabinet spokesman said the 25-mile area would extend from the border.

President Reagan joined the six other leaders attending the economic summit meeting in Versailles near Paris in issuing an urgent appeal for a cease-fire in Lebanon.

In Damascus, the Syrian military command said, "The invading Israeli forces in southern Lebanon have advanced toward our forces and, after taking up positions just facing our own, they began shelling our forces." Later Radio Lebanon said that Syrian artillery had opened fire on the Israelis. Earlier, the Israeli Cabinet said, "The Syrian Army will not be attacked unless it attacks our forces."

Saudi Arabia's King Khalid sent messages to President Reagan and other world leaders calling for "quick intervention" to halt the Israeli "massacre of innocent Lebanese civilians and Palestinian refugees."

In the Soviet Union, the government news agency Tass denounced Israel for launching what it called the "fifth war against the Arabs."

Egyptian President Hosni Mubarak told reporters that the situation in Lebanon would dim the prospects for peace in the Middle East.