"A dozen families smashed the door and just moved in," the upper-class woman recounted to a friend over the telephone. "We didn't argue because they were accompanied by armed leftist militiamen."

Here was a panicky story repeated countless times as tens of thousands of refugees fleeing the Israeli invasion of southern Lebanon continued to pour into this capital already swollen by a half million displaced persons from the 1975-1976 civil war.

Their belongings piled into often decrepit taxis or pickup trucks, the new refugees drive down Beirut streets searching for shelter.

Luxury housing seems to be a favorite target.

Many such apartments stand empty because their Lebanese owners are demanding sky-high rents in the $15,000-a-year-range.

That kind of housing once was favored by the Western business community which has never returned to Beirut in any great numbers since the civil war.

Many Beirut residents find the present invasion all too reminiscent of the anarchy which prevailed during the fighting.

For example, the new squatters often are accompanied by militiamen from various Palestinian guerrilla or leftist Lebanese groups which had largely disappeared from city streets since the 30,000-man Syrian peacekeeping force arrived in 1976.

'On several occasions, arguments over empty housing have degenerated into shootouts.

As it was during the war, the Lebanese police is of no discernible help. Calls to police stations are often answered by the sole policemen on duty.

Members of the 30,000-man peacekeeping force reportedly slapped and beaten up Beirut residents who complained about the squatters.

The Lebanese government has turned down an American offer of prefabricated housing for the refugees because it wants them to go back home.

Some home owners have taken to sending members of the family to empty apartments in the hopes that their presence will deter squatters.

A young woman was offered a year's free rent in exchange for occupying an empty apartment.

Other Beirut residents simply have battened down doors to their homes and prayed.

Social life has dropped off because people fear that they will find their homes occupied if they dine out with friends.

One businessman went skiing at the nearby resort of Faraya and returned to find his office occupied and telexes and other equipment looted.

As happened during the civil war, organized gangs help refugees break into apartments, then loot what they can as a kind of unofficial finders foe.

Although many refugees are arriving with few or no belongings, the Lebanese Red Cross provides no blankets.

The official reasoning is that the refugees would only sell the blankets just as they sell sugar, flour and other staples that are provided free and later turn up in neighborhood groceries.

In the once upper middle-class neighborhhod of Kantari, a wartime refugee favorite, neighborhood officials estimate as many as 3,000 people arrived from the southern village of Tibinie which fell to the Israelis yesterday.

They were luckier than two taxi-loads of Tibnine villagers who were ambushed before dawn on the coastal highway at Aadloun about halfway between Tyre and Sidon.

The taxi was riddled with machine-gun bullets, the other was hit by a the fin of a rocket with Hebrew lettering on it. As many as 20 villagers - most of them women and children - were killed. A survivor spoke of seeing uniformed men shooting from the beach which parallels the road.

Apart from the commando raid at Aadloun, which is located 22 miles north of the Lebanese-Israeli border, Israeli military activities were largely confined to securing their conguest of the six-mile-wide strip of territory next to the border. Along with Tibnine, the Israelis pushed Palestinian guerrillas out of five more strong-holds inside the "security belt."

Palestinian guerrilla spokesmen said yesterday their forces were conducting hit-and-run attacks in Israeli-held areas.

In an apparent reference to the Aadloun attack, Israeli Defense Minister Ezer Weizman said in Tel Aviv that "one of our crack units made an ambush north of Tyre and, according to their report, we killed one of the (Palestinian) commanders."

Weizman did not identify the commander except by saying that he was a top associate of George Habash, leader of the radical group known as the Popular Front for the Liberation of Palestine.

Weizman added: "There was no intention to inflict civilian casualties. Some such casualties may have occurred in the course of the ambush."