Waves of Israeli warplanes bombed heavily populated Palestinian neighborhoods in Beirut and targets in southern Lebanon today, killing at least 123 persons and wounding hundreds more in Israel's most devastating attack here since its invasion in 1978.

The midmorning raids here were the first on the Lebanese capital in seven years and they left Palestinians and Lebanese dazed and fearful of more attacks as Israeli leaders announced they would step up their pressure in an attempt to halt guerrilla actions against the Jewish state.

At the same time, the Palestinians resumed their shelling of villages in northern Israel, firing rockets at Metullah, Kiryat Shemona and other sites, apparently in retaliation for earlier Israeli bombings.

The widespread raids, which also hit along the Mediterranean coast and across southern Lebanon, came as special U.S. envoy Philip C. Habib met ith officials in Saudi Arabia in his effort to defuse the Syrian-Israeli missile crisis. Habib reportedly postponed a planned flight to Damascu today after learning of the Israeli raids.

It appeared here today that the new crisis between Israel and Lebanon, after six days of bombing raids, may have overtaken the two-month-old confrontation between Israel and Syria over the deployment of Syrian antiaircraft missiles in Lebanon.

The heaviest casualties in Beirut apparently came when bombs flattened a seven-story building where, according to Palestinian spokesmen, families of guerrillas lived. Six hours after the raids, bullodozers and masked rescue workers were still uncovering bodies in the burning rubble as agitated, gun-toting guerrillas roamed about, trying to keep order.

The raids along the coast, coupled with Israeli gunboat shelling, cut the major north-south road in the region, severely hampering Palestinian supply activities and leaving much of northern Lebanon without direct access to its major fuel supply facility near Sidon. Israeli warplanes streaked in low and in pairs from the Mediterranean across the capital, rocketing and bombing for more than a half hour in a clear attempt to hit the main headquarters of several Palestinian guerrilla groups clustered near the crowded downtown area of Beirut and the Arab University.

Residents said the Israelis had never before bombed so close to the center, although they have attacked Palestinian refugee camps on the outskirts a number of times in the past. It was an awesome spectacle, which many watched from the balconies of their apartments and offices.

There were conflicting reports of four to six passes by the warplanes, which were said by the Christian Phalangist radio to have included U.S.-built F4 Phantoms and F15s. The radio warned that a "catastrophic tragedy" was unfolding on Lebanon.

The Israeli military command said the jets had hit the headquarters of Fatah, headed by Yasser Arafat, the largest group within the Palestine Liberation Organization, as well as that of the smaller Marxist Democratic Front for the Liberation of Palestine, headed by Naif Hawatmeh.

This claim could only be partly confirmed by reporters who visited the Fakehani quarter, which houses the headquarters of these and several other Palestinian groups.

Palestinian sources said one of the five buildings bombed contained offices of the Democratic Front and that it was about 60 percent destroyed. But they emphaticaly denied that any of Fatah's offices had been affected by the bombing.

They said most of the bombs and rockets fell on residential quarters and they took reporters to see the seven-story building that had completely collapsed, apparently killing most of those inside.

As agitated Palestinian guerrillas roamed about the scene trying to keep order, a Democratic Front spokesman angrily told a group of reporters: "I hope you will tell your readers that the Palestinians hold the American government and Reagan administration responsible for this."

Reporters were not taken to see the other buildings reported bombed in Fakehani, making it impossible to assess the extent of the damage.

As of late afternoon, the Palestinian news agency WAFA reported that the death toll had reached 123 but later the Phalangist radio put the figure at 150.

By far most of the casualties were here in Beirut, where, in addition to the Fakehani neighborhood the Israeli warplanes also hit several Palestinian refugee camps on the southern outskirts of the city, including Sabra and Shatila as well as suburban Uzai and Sporting City.

The Phalangist station said three Syrian soldiers were killed in the bombing and 20 wounded, but this report could not be confirmed.

The Israeli warplanes also hit a refugee camp outside Tyre, 50 miles south of Beirut, and bombed the main coastal highway in several places between Damour and Sidon, thoroughly disrupting traffic between the capital and the south.

I watched as Israeli jets came in from the sea at Saadiyat, a few miles south of Damour, to blast the roadbed a half-mile ahead of us with bombs. When we reached the scene several minutes later, we found shell and bomb craters that had torn up 200 yards of road. Four cars lay destroyed in the rubble of broken pavement but their passengers had apparently escaped.

Nearby, the tracks of a railroad built by the Ottoman rulers in the 19th century were only twisted and jagged pieces of steel.

South of Sidon, near the port facility of the Trans-Arabian Pipeline Corp., which carries oil from Saudi Arabia to the Mediterranean, we suddenly found the road under fire from an Israeli gunboat a half-mile offshore.

In the virtually deserted area, the gunboat fired at will at the roadway. Shells landed within 50 feet of us as we tried to take shelter in the terminal and a small, secondary pipe was hit, but the gunship appeared to be firing at the highway, not the oil facilities.

Israeli planes also knocked out a bridge at Damour, forcing traffic between the capital and the south along the coast to take a trotuous four-hour detour through the mountains, causing mammoth traffic jams in the tiny vilage lanes.

Yesterday, Israeli warplanes destroyed six bridges in the south in an apparent attempt to isolate Palestinian strongholds there from their supply centers on the coast.

But after today's raids on the main coastal road, it appeared that Israel was attempting to disrupt all traffic to the south, Lebanese as well as Palestinian.

One of the immediate consequencs of cutting of road links between Beirut and the south was a gasoline shortage. Long lines were forming at stations all along the road from Sidon to the capital and many of them had already closed.

One of Lebanon's main refineries is at Zahrani south of Sidon, but with the main coastal road now cut in several places it may prove impossible to get gasoline to the capital.

Today's Israeli bombing seems to have done little to impair official Palestinian morale, despite the high casualties among civilians.

Mahmoud Labadi, the oficil PLO spokesman, issued a statement tonight saying "The one thing they will never break is our will. We will go on resisting no matter what kind of sophisticated weapons they use. We will survive no matter what Begin or Reagan want."

Both the Palestinians and Lebanese say they are convinced that Israel has obtained the blessing of the U.S. government for its attacks on guerrilla positions in Lebanon. Indicative of this viewpoint was the comment of the Democratic Front spokesman, Daoud Talhami, who said as he stood with reporters before the smoldering ruins of the seven-story building in Fakehani, "The Habib mission has given the Israelis the green light of this."

[In Washingon, Hatem Hussaini, director of the PLO information office sad that the "mission of Habib is now totally in runis. The raids today destroyed any American effort to appear neutral or mediate."]