Guerrilla-driven bulldozers and dump trucks were back at work with a new urgency here today, piling earthen barriers to cut the main seaside thoroughfare and other roads from the potential invasion by Israeli troops who now surround predominantly Moslem West Beirut. Other guerrillas laid mines.

The Israelis have insisted they will not attack or occupy Beirut. But since linking up last Sunday with their local allies, the predominantly Christian Lebanese Forces who rule the eastern part of the city, the Beirut that Israel has declared off-limits has been redefined repeatedly and has shrunk steadily.

What already had been two cities, divided since the civil war in 1975 between Christians and Moslems, now has split again, like a cancerous cell, into three.

The civil war "Green Line" still exists, drawn north to south through the center of the city from the port to the foothills. In between is a no-man's-land where rival troops of the Christian Maronite militias, in the east, and Moslem and Palestinian groups, in the west, man the ruins of office buildings, movie houses, department stores and cheap hotels.

Now, thanks to the Israelis, there is a new invisible line running well within the city's formal limits in West Beirut.

That line divides the west into two parts--what is supposed to be the "safe" part of the city, kept free from Israeli bombs and troops, and the southwestern portions where the Palestinian refugee shantytowns--and those of the many Shiite Moslem poor as well--are located.

Yet since the fighting started the Israelis have bombed and shelled not just the Palestinian and Moslem camps within West Beirut, but also within the supposedly safe limits of the new dividing line.

Inside that ever-diminishing, supposedly "safe" area are to be found Beirut's classic traffic jams. On closer inspection they are caused by the hundreds of thousands of Lebanese and Palestinians who have deserted their shantytowns now pounded by the Israeli guns, leaving the Palestinian fighters to defend their homes. The traffic tie-ups increased today in what appeared to be a panic motivated by reports that a 48-hour cease-fire, agreed to by the Israelis to allow time for political negotiations over the city's future, would expire Sunday.

The refugee families are camping out in schools, gardens, requisitioned apartments and in lobbies of buildings.

Other nervous guerrillas and their Lebanese allies of the so-called "joint forces"--many of whom were defeated in earlier battles farther south--are ensconced in buildings in this inner West Beirut.

Key defensive positions are in buildings along the once-graceful, winding seafront and throughout the perimeter, including the inland areas far from both the seafront and the Green Line.

Despite the terrible toll in civilian casualties that Israeli ground assaults exacted in Tyre and Sidon farther south along the coast, the Palestinians and their allies seem determined to fight among the civilians throughout West Beirut. They believe their best hope lies in hit-and-run tactics against the Israelis, who both outnumber and outgun them.

The Israelis' answer to such tactics has been bombing and heavy artillery.

This morning, under the thud of incoming Israeli artillery, fear gripped West Beirut. Within the western city's 10 square miles, residents are convinced they know where the Israelis will attack. The danger zone begins at Ramlet Baida, once an upper-class neighborhood with a beach to rival Rio's. Then it spreads through Jnah, across the much-bombed sports stadium to Fakhani and the Arab University areas where the Palestine Liberation Organization offices are.

By late morning, cars were lined up four abreastfor three quarters of a mile waiting in the hot sun to pass through Lebanese Army checkpoints into the east at one of several access points.

An excited, pregnant young woman rushed into a taxi office in late morning. She had just received a telex message from her pilot husband in Dubai ordering her and their 3-year-old son to leave immediately.

"Wait until later," she was advised gently.

Once beyond the Corniche Mazraa, a wide avenue that serves as the invisible frontier, traffic thins. The only people on the street areguerrillas manning truck-mounted antiaircraft guns parked under bridges or trees for cover.

But it is the silence that both impresses and depresses. For these were among the most lively and animated neighborhoods in the Arab world, full of noise, bustle, aggravation and life.

Mahmoud Labadi, the PLO official spokesman, is all but alone in an office that once was full of assistants and journalists from around the world hoping for interviews and guided tours.

The phone rings, out come the PLO arguments, well-marshaled, spoken with conviction, invoking increased Soviet, French, Austrian support. The Arab world is finally waking up and rallying; Iranian volunteers are about to arrive.

On the walls are photographs of PLO leader Yasser Arafat and Vietnamese revolutionary Ho Chi Minh staring out at posters showing a map of Palestine with the caption "Until final victory."

The phone call ends. "Radio Cyprus," Labadi explains, almost apologetically. He frankly admits that he and the leadership fear that nothing is going to stop the Israelis from making their onslaught.

"If they can kill everyone, they will never hesitate," he remarked, "but we will make them pay a high price. Our only hope is international pressure."

He issues a pass to PLO territory, hesitating, with a smile creeping over his face before asking a visitor, "Should I say you are a friend of the revolution or just a correspondent?"

Down in the street, cordoned off at either end to guard against the car bombs that only two weeks ago seemed the most immediate threat to the PLO, a garbage truck is at work. In itself that is a small miracle for at the best of times in West Beirut garbage is not collected regularly.

Today was the first time since the Israeli invasion that the trucks have been at work.