The western section of this city, once a fashionable jewel of the Middle East, now undoubtedly has the world's meanest streets, ravaged by the latest fighting with Israel and by years of civil strife.

In the newest round of destruction visited on Beirut, Palestinian guerrilla positions are the professed targets. But the shells of Israeli planes, artillery and gunboats do not discriminate, and homes, apartment buildings, hotels, factories and shops in the western sector have been destroyed.

In the Sabra Palestinian refugee camp on the southern end of the city, Fayad, 38, recently returned to inspect the one-story cinderblock house he built 15 years ago.

The rubble-strewn living room was visible through a gaping hole blasted in the front wall three days before by an Israeli artillery shell. In front of the house, a few feet away from where the shell struck, sat a sofa covered by an old blanket. Shrapnel had torn a dozen holes in it.

"We were sitting right there on that sofa," Fayad said as he stood before the house with a few neighbors. "Fortunately we left five minutes before the shelling started."

As he spoke, his wife emerged after rummaging through the debris of her home and began shouting angrily at a reporter. "You come always to take pictures and then send them to Israel," she screamed. "There's a conspiracy against us--by the reactionary Arab regimes and by the whole world."

The destruction reaches to the opposite side of West Beirut, in a street just off the Avenue de Paris that runs along the rocky edge of the Mediterranean Sea. There a shell fired from an Israeli gunboat landed 10 yards in front of the apartment building where this reporter lived four years ago.

But accompanying the physical destruction from the Israeli attacks is a pervasive sense of lawlessness. Shooting that has nothing to do with the Israeli invasion can be heard daily in West Beirut.

West Beirut is suffering from the decay brought by years of civil war and lawlessness at least as much as it is suffering from Israel's shelling. Hamra Street, once the city's most fashionable, is now lined with shuttered shops and littered with burned-out or abandoned hulks of cars and heaps of uncollected, fly-infested refuse.

Just the other day, reporters sitting in an anteroom waiting to see former prime minister Saeb Salam suddenly had to dive for cover when a car came to a screeching halt outside the house and gunmen began spraying guards with automatic weapons fire. The guards fired back, and one person was shot in the foot before the car sped away. Later, the shooting was blamed not on the Israeli assault here but was described as an act of vengeance by a former guard who had been dismissed.

Despite the random violence, the old damage from the 1975-76 civil war and the new damage from the Israeli invasion, many of the 600,000 inhabitants have stayed in West Beirut to protect their homes and belongings from squatters and looters. Much of the city remains far from devastated. Modern apartment and office buildings still stand not far from others ruined by the civil war, and new construction has been started since then, although virtually all of it is now abandoned.

But the violence continues. Guerrillas and militiamen, far from manning lines of defense against an Israeli attack, can be seen ambling down Hamra wearing their motley assortments of olive drab and camouflage uniforms draped with various war paraphernalia and carrying the ubiquitous Kalashnikov automatic rifles with the distinctive banana-shaped clip.

At one point last week, armed groups battled for about 30 minutes with machine guns and at least one rocket-propelled grenade launcher in a side street next to the Commodore Hotel, where most foreign correspondents stay. When some reporters tried to go outside to see what was happening, excited militiamen backed their jeep up to the hotel's glass front doors and trained their rear-mounted, .50-caliber Dushka machine gun on people in the lobby, threatening to open fire.

According to different versions, the battle was either a dispute over gasoline--which is in short supply--or over refugees' moving into an apartment.

Saturday in front of the French Embassy, which has been a target of bombings that have killed a number of people and blown out windows in the now-vacated modern office building across the street, a car sped by and, for an unknown reason, gunmen inside fired at and missed a man walking on the sidewalk.

A car bomb, one of at least a dozen since the Israeli invasion, killed two persons and wounded 21 in a downtown West Beirut street recently. The Palestine Liberation Organization later executed three Lebanese Shiite Moslems who confessed publicly to two bombings and said they had been working for the Israelis. However, longtime Beirut residents believe some of the bombings are the result of failure to pay protection money to militiamen.

It seems as though almost every other man carries some kind of weapon. Various war vehicles--from jeeps mounted with machine guns or recoilless rifles to armored personnel carriers and at least one old Soviet T34 tank--can also be seen on the nearly deserted streets. They are not, however, in sufficient numbers to pose any serious obstacle to a wholesale Israeli assault.

Despite the violence that has punctured life here since the civil war, the shelling accompanying the Israeli invasion has heightened the suffering among the Palestinians and the Lebanese in the western section of the city.

In a Lebanese working-class neighborhood on the southern edge of West Beirut, Osman Habbal last week surveyed the smoldering remains of his family's rope and cable factory. The one-story factory had been hit early Wednesday by what neighborhood residents claimed were Israeli phosphorous bombs.

An adjacent seven-story apartment building was completely blackened, and residents said 25 houses had been burned out.

"We think the Israelis must have thought this was an arms depot," Habbal said. "But as you can see, there are no guns here."

On a tour through Sabra and the neighboring camp at Shatila, a PLO official led a visitor through a maze of alleys between multiple-story buildings that were vacant but still intact.

The dead end was the rubble of a two-story house that the guide said had been hit by an Israeli shell fired from a position in hills to the east. Broken furniture, men's and women's shoes, clothes, a plastic bottle of Johnson's baby powder and an assortment of other items lay amid the rubble. In a room next door, decorations from a party still hung from a chandelier.

The PLO guide denied that there had been military positions in the refugee camp, but he added, "Now the camps are empty, and there are military positions."

"There are no civilian areas now." he said. "Anyway, Israel will not differentiate between civilian and military areas.