On the east side of this divided capital city, a jeep-load of Israeli soldiers clips along the coastal highway. They are hardly noticed by blase beachgoers as they drink in the sparkling, holiday atmosphere of the Maronite Christian enclave.
On the west side, gunmen in red berets eye each car suspiciously, interrogate passengers, guard the approaches to the Palestinian quarters. Their comrades plant mines in the trash-strewn streets and otherwise seal themselves off from the outside world.
Unlike wide-open east Beirut, west Beirut seemed to have the unhappy choice Sunday of becoming either an urban battleground or an urban prison.
If the Israeli Army attacks the Palestinians holding out on the west side of town, the ensuing violence will be costly all round. Only the most idealistic Palestine Liberation Organization (PLO) members believe a determined Israeli assault could be turned back.
But, with an on-the-ground cease-fire still in effect at time of writing, the prospect for an immediate Israeli attack seemed to have diminished. Many Lebanese and Palestinian intellectuals now believe that instead of an all-out Israeli final assault on west Beirut, the situation will evolve in this manner:
* West Beirut will be increasingly cut off from the outside world -- becoming a Palestinian city-state. Uncomfortably surrounded by Israeli forces and by the Maronite Christian Phalangists, the PLO guerrillas will make themselves and anyone who stays here virtual prisoners.
* Slowly the armed Palestinian resistance here will be worn down. Little or no new ammunition will get into the city. Flare-ups along Israeli-PLO and Phalangist-PLO confrontation lines will wear away at guerrilla strength, and guerrillas will become weary with long lulls, brief fights, short-term cease-fires, and nowhere to go for rest.
* Pocketed in Beirut, PLO leaders such as Yasser Arafat will be unable to exert their customary leadership of the Palestinian cause. At first, the trapped leaders will be something of a cause celebre in the Arab world. But in time local leaders will emerge in the West Bank, Gaza, Jordan, Syria, and elsewhere.
Meanwhile, if it indeed avoids a final assault, Israel will be spared world criticism for destroying west Beirut and ruthlessly going after the now-contained and virtually harmless (to Israel) PLO. Moreover, the United States, which in this part of the world is closely identified with Israel, will be spared Arab censure for otherwise countenancing the destruction of west Beirut (the only Arab capital Israelis ever would have entered by force) and of the PLO.
''An Arab can be beaten militarily as much as you want,'' a veteran Arab editor here notes. ''But spare him his humiliation. Going into west Beirut would be humiliation.''
There are others here -- diplomats included -- who see the massed military might of the Israeli Army and the Phalange as having an impetus of its own. With Israel and the Phalange this close to crushing the PLO, they reason, can they hold back?
Diplomacy was still at work June 20 to try to achieve an end to hostilities. These negotiations -- taking place both among Lebanese leaders and internationally -- have as their basic aim the devising of a formula whereby the PLO could give up its weapons and yet be assured protection from its enemies.
This would mean agreements, in order: among the Lebanese factions; between the Lebanese and the PLO; and between the Lebanese and Israelis.
An encouraging sign was that the Lebanese Council of National Salvation met for the first time June 20 with all the Lebanese leaders that President Elias Sarkis invited in attendance. And in Israel, the cabinet decided to extend the cease-fire, saying it was doing so at the behest of US envoy Philip C. Habib and Lebanese leftist leader Walid Jumblatt.
Even so, the PLO's public statements show that it is not willing to concede its military arm - a contrast with hints last week by a top PLO official that preserving the PLO politically, not militarily, was what mattered.
A top Phalange official said he had detected a hardening of the Palestinian attitude since last week and he was pessimistic about a political settlement in time to avert an Israeli attack:
''Now we don't see how the Palestinians will give up their arms, dismantle their infrastructure, and see their empire disappear.''