Israeli forces inflicted severe damage in practically every West Beirut neighborhood today as their artillery, armor, gunboats and, finally, the Air Force unleashed the heaviest assault yet in the eight-week-long siege of the Lebanese capital.

Beginning just before midnight Tuesday, and lasting through nightfall tonight, the fierce assault shook the ground of entire neighborhoods with high explosives, white phosphorous or cluster-bomb projectiles. After dark, an uneasy lull took over, punctuated by intermittent explosions that continued throughout the night.

Today's land, sea and air attack apparently was designed to impose additional pressure for withdrawal of guerrilla forces still entrenched in West Beirut, and to consolidate ground positions gained last night in a three-pronged assault from the south and across the Green Line separating the eastern and western sectors of the city.

Throughout the day, Palestinian gunners and their Syrian and left-wing Lebanese allies in West Beirut fired back rockets, mortars and artillery at Israeli positions in predominantly Christian East Beirut, reportedly causing some casualties.

According to Palestinian officials, negotiators working early this morning as the barrage was in progress were reported nearing final agreement on a compromise plan designed to head off just such Israeli military punishment and arrange for the departure from the city of Palestinian guerrillas.

The most successful of the three post-midnight pushes was an Israeli advance of more than two miles north along the seacoast. It left Israeli forces in a Lebanese Army barracks strategically located near Palestinian refugee camps and little more than a mile short of the main east-west boulevard, Corniche Mazraa, separating West Beirut proper from the suburbs and refugee camps in the south.

The Israelis also were even closer there to a strategic coastal crossroads, controlling the ramshackle suburb of Ouzai lying between the sea and the airport runways occupied last month.

An Israeli officer in East Beirut said Israeli units were already "mopping up" the Palestinians and their Lebanese allies who now are blocked on three sides, with only the sea providing an outlet.

For the last 90 minutes before nightfall tonight, Israeli warplanes bombed Palestinian positions in the sector around the barracks in an apparent effort to close the trap on the guerrillas and tighten the noose around their refugee camp strongholds.

More difficult to assess were the after-midnight drives across two of the three main crossing points--at the Museum and the Beirut port--separating predominantly Moslem West Beirut from the capital's Christian sector to the east.

Both thrusts may have been feints, according to military specialists, or the Israelis may have been frustrated in attempts to slice through the Corniche Mazraa to the sea and link up with other units moving north from the airport and south from the port.

The main armor thrust, across the Museum checkpoint, quickly fanned out in the adjacent race track where the invaders were locked in fierce fighting many hours later with Palestinian guerrillas, according to Palestinian sources.

An Israeli officer in East Beirut claimed at midday that Israeli troops controlled an area 60 yards wide and 600 yards long near the Museum crossing point.

According to Washington Post correspondent William Branigin, who traveled today to the crossing point located in a heavily damaged sector that has served as the dividing line between East and West Beirut since Lebanon's 1975-76 civil war, the area has never seemed more of a no man's land.

None of the Israeli tanks reported to have crossed through last night was visible. According to a Palestine Liberation Organization guerrilla, the Israelis advanced a few hundred yards through the no man's land during the night, then turned left into the vast race track known as the Hippodrome, separated from the street by a long stone wall.

Wearing an olive-drab uniform with a Soviet-style helmet and vests packed with extra ammunition, the guerrilla said fighting was still going on in and around the race track, formerly held by Syrian and Palestinian troops, and at the Palais Mansour, the parliament building controlled until yesterday by the Lebanese Army.

According to another PLO guerrilla, the guerrillas, Lebanese Moslem militiamen and Syrian troops defending West Beirut have fought back against Israeli tanks by running up to the earthen barriers in the street and firing bazookas. The fighter said four Israeli tanks had been knocked out in this manner in the battles on this southern edge of West Beirut early today, but the claim could not be confirmed.

An officer of the Syrian-controlled Palestine Liberation Army told Branigin that the Israelis had not made headway. But it was unclear whether this was due to stiff resistance or because a reported thrust last night at the port had been a feint, after which the Israelis had withdrawn.

The Israeli armor drive reached the strategically located Fattal office building just beyond the port in the former West Beirut business center ruined in the civil war. But eyewitnesses at midmorning watched some 40 tanks withdraw to the Christian side of the port, which they occupied early last month.

No shooting could be heard from the port area late this afternoon, Branigin reported, but shells continued to whistle in and explode in nearby neighborhoods.

By midafternoon, Wafa, the Palestinian news agency, was claiming victory. It said its forces had destroyed 21 Israeli tanks, 14 armored personnel carriers, inflicted more than 100 casualties on the two Israeli divisions committed to the battle, one armored, the other mechanized.

Wafa provided no mention of Palestinian losses. But correspondents watching the Israeli shelling against Palestinian positions were convinced that casualties must have been considerable.

In Damascus, a military spokesman said that 21 Syrian soldiers had been killed or wounded in fighting Wednesday against Israeli forces in Beirut, Reuter reported.

The spokesman added that while Israeli forces had achieved "some advance," they had been prevented from reaching their goals. Israeli losses included 40 tanks, one armored personnel carrier and at least 150 killed and wounded, he said.

Corrrespondents in a high-rise building watched as Israeli artillery and naval gunfire laid down a saturation barrage over several square miles that appeared to leave nothing untouched before the many hundreds of explosions were obscured in a cloud of thick smoke.

Reports of progress in the negotiations had come only hours before, as Palestinian officials said that, for the first time since the arduous talks began more than six weeks ago, the guerrillas agreed to drop demands that the Israeli Army withdraw a token distance from its present lines, the officials said.

Instead, the officials said they had agreed to leave once an international force was in place alongside the Lebanese Army around their lines, principally in the now-devastated refugee camps.

Part of the plan called for U.S. guarantees that neither the Israelis nor their right-wing Christian allies would interfere with the transfer of the departing guerrillas or the well-being of Palestinian civilians remaining in the camps.

All the departing guerrillas would leave with their individual weapons along the Beirut-Damascus road. The bulk would continue straight for the Syrian border. For those who for ideological reasons refused to go to Syria, the officials said, provisions were to be made for them either to fly from Rayack Air Force Base in the eastern Bekaa Valley or sail from the northern Lebanese port of Tripoli.