Looking at first like a pleasure yacht in the distance, the Israeli warship sat in the calm Mediterranean half a mile offshore.

Suddenly, without even a puff of smoke for a warning, there was a deafening explosion as a shell slammed into the ground, sending a group of Western journalists scurrying for cover in the basement of the Trans-Arabian pipeline terminal building.

All day Friday, while Israeli warplanes bombed Beirut and other regions of Lebanon, the deadly little vessel fired away intermittently at the terminal and the coastal road running through the Tapline grounds, cutting traffic. After a few rounds, it would retreat out to sea, only to return a little while later to shell again, with only an occasional burst of machine-gun or light artillery fire opposing it.

The story of the gunboat seems all too symbolic of the state of this war-wrecked skeleton of a nation. Perhaps the strongest impression this reporter has had during a week of violence here is just how wide open and defenseless Lebanon and its Palestinian guerrilla encampments are to Israeli air and sea attacks.

The helplessness seems summed up in the picture -- often witnessed here this week -- of embittered Palestinian guerrillas firing wildly with their AK47 assault rifles into the air in the slim chance of hitting a passing Israeli jet.

For the past 12 days, Israel has shown the world the devastation it can sow here with its airpower, bombing heavily populated areas of downtown Beirut, cutting bridges throughout southern Lebanon and hitting other targets of its choosing, such as the Zahrani refinery depots.

The result has been more than 1,150 casualties, including nearly 300 deaths, the vast majority of them Palestinian and Lebanese civilians; the partial severing of southern Lebanon from the rest of the country; a potentially crippling gasoline shortage; and another full-scale national crisis.

In the same period, Israel suffered five dead and about 50 wounded from Palestinian rockets and artillery fired on villages across northern Israel.

The Israeli military has made much of the half-dozen batteries of SA6 antiaircraft missiles that Syria brought into Lebanon's eastern Bekkaa Valley in late April. Prime Minister Menachem Begin's government said the missiles curtail Israeli operations against the Palestinians and pose a threat to Israel's security.

The Syrians have 22,000 troops here under the auspices of an Arab peacekeeping force. They said they sent in the missiles to protect the troops after Israeli warplanes shot down two Syrian helicopters during fighting with Lebanese Christian militia forces.

But there was no indication this week that these missiles were any deterrent to Israel's might. Indeed, aside from the Bekkaa region, Israeli warplanes and gunboats seemed to have pretty much a free run of all Lebanon, able to strike where and when they will with near total impunity.

Even the more sophisticated weaponry in the Syrian and Palestinian arsenal -- SA7 and SA9 missiles and antiaircraft artillery emplacements around the capital -- seem easily evaded by the American-built Israeli warplanes, with the help of ultramodern devices such as balloon-floated heat deflectors.

Syria claims to have shot down two Israeli jets during Friday's attacks on Beirut and southern Lebanon, but there has been no confirmation of this report by Israel or any other source.

But latest Israeli attacks have set off an outcry among the Lebanese themselves, who are demanding that the government and other Arab nations do something to defend the country against Israel's promised escalation of war against the Palestinian guerrillas here.

While it is too early to say what the outcome will be, there is a good chance the raids will serve to accelerate the flow into Lebanon of heavier arms to shield Lebanon and the Palestinians from Israel. Already, Lebanese leftist groups have asked for SA6 missiles, like those Syria brought into the Bekaa, to defend the capital from further Israeli raids.

At the same time, Libya was reported here Sunday to have offered "total aerial protection" to Lebanon and the placing of its military forces at the disposal of the Lebanese government. The offer was made by Libya's chief diplomatic representative here, Abdul Kader Gouka, in a meeting with Lebanese Premier Shafik Wazzan, according to the pro-Libyan daily As Safir.

Libya had earlier pledged to provide Lebanon with SA6 missiles, if it wanted them, and has already delivered the shorter-range SA9s and SA7s to the Palestinians.

What Israel would do if Libya did send SA6 missiles remains to be seen, but the Begin government has repeatedly insisted Syria must remove such milliles from the Bekkaa Valley and promised to destroy them if they are not taken away peacefully.

There are urgent calls for a joint Arab strategy to halt the Israeli attacks.Saudi Arabia has demanded that a July 25 meeting of an Arab committee originally set up to seek a solution to the Lebanese internal political crisis, should deal first with the Israeli challenge. Other Arab leaders are calling for Arab League defense ministers to hold an emergency session.

This may not give birth to a joint Arab strategy for the defense of Lebanon. But it may well lead to the sanctioning of greater Syrian and Libyan involvement here as well as the arival of ever more sophisticated arms from the Soviet Bloc.