A week ago, an alliance of Lebanese leftist Moslems and Palestinian guerrillas ran this mountain village 12 miles southeast of Beirut under the aegis of the Syrian Army. They manned checkpoints, raced around the streets in jeeps mounted with machine guns and occupied a large stone house on a hill.

Today the house is the local headquarters of the right-wing Christian Phalangist militia, installed here after Israeli troops took control.

Despite Israeli and Christian claims that they have no military cooperation, it is becoming clear that the Phalangist Party, Lebanon's predominant Christian armed faction, is taking advantage of the Israeli invasion to enlarge its area of control in Lebanon without having to do any serious fighting.

Besides taking over this former Druze stronghold, the Phalangists moved into the key towns of Jamhour and Bhamdoun on the Beirut-Damascus road this weekend on the heels of an Israeli thrust that drove out the Syrians and Palestinians.

While the presence of the Phalangists and Israelis is generally welcomed by Christian residents, others, notably Druze and Moslems, are decidedly nervous. They say that Phalangist militiamen have been rounding up young men for interrogation about their political sympathies and that some have disappeared. Most of the leftist militia and Palestinian guerrillas fled without a fight, residents say.

Many of the Syrian and Palestinian occupiers of Jamhour and Bhamdoun also abandoned those towns in disarray under the Israelis' heavy shelling and air strikes preceding their advance. However, hard street fighting took place, especially in Bhamdoun, residents said. Today the three towns show the devastating effectiveness of the Israeli war machine. Many buildings have been demolished by artillery and tank fire and air bombardment.

Many bodies of Syrian soldiers killed in the fighting have been piled up and burned, but some still lie where they fell, bloated and blackened under the summer sun.

The body of one Syrian still smoldered by the side of the road at the entrance to Jamhour today as a militiaman stood nearby. Four others were rotting in a private car that had been blasted by gunfire, and several more lay uncollected on the road.

Israeli soldiers said the terrible destruction they wrought was unavoidable because the Syrians and Palestinians had taken up positions in buildings among the civilian population.

"For the Israeli Army there was no choice; we had to shoot," said one disheveled young soldier. Waving his arm at the devastated buildings around Bhamdoun's central plaza, he added, "We are very sorry about all these things you see."

Despite the destruction of their own property, Christian residents of Bhamdoun expressed satisfaction that the Israelis had chased out the Syrians and Palestinians.

Although stationed here as Arab League "peace-keeping" troops, following the 1975-76 civil war, the Syrians have come to be regarded by many Lebanese as an occupation army.

"We're very happy to see the Israelis," said the manager of a damaged hotel.

"Now we feel free," said a Christian inhabitant of a shell-scored stone house. However, both declined to give their names, saying they could not be sure what might happen in the future.

Their feelings were shared by a Moslem Shiite merchant in a village near Aley, who said the Syrian presence had brought "anarchy." Standing in front of his general store next to a former Syrian checkpoint, he complained that citizens' houses had been arbitrarily confiscated and property seized. He added that the Syrian next door had left without paying a tab of about $200.

In Aley, however, several residents expressed apprehension at the change of masters.

A relief worker of the Druze sect said he saw the Phalangists arrest about 30 persons and take them away. He said some residents, including Druze religious leaders, were interrogated at gunpoint or beaten. He said he was held briefly and released after his identity papers showed he was a medical worker.

"I feel this is not a real peace," he said. "We want the army of Lebanon to take care of the whole country."

One of the Phalangist militiamen now occupying the former headquarters of the leftist National Movement said that if any of their Palestinian or Lebanese Moslem foes are found here, "we kill them." He said that although there had been no street fighting here, some persons already had been killed since the Phalangists entered the town yesterday.

In predominantly Christian East Beirut, a senior Phalangist official said the militia did not seek to expand its zone but was merely "filling a military vacuum" because the Lebanese Army was not yet able to do so.

"When we have a political decision to bring the Lebanese Army in to assure security, we will immediately be ready to withdraw and let the Army assume its responsiblities," said Karim Pakradouni, a member of the Phalangist Politburo and a close adviser to its military commander, Bashir Gemayel.

The Lebanese Army disintegrated during the civil war between Christian militias and an alliance of Palestinians and Lebanese Moslems. Despite subsequent efforts to reconstitute it, the Army has remained weak compared to the militias and tends to splinter along factional lines during a conflict.

According to Pakradouni, the Phalangists have three major demands: complete disarmament of the Palestinians, withdrawal of Syrian forces and installation of a strong central government in Lebanon. He claimed that a consensus on these objectives now has been reached on the international, Arab and Lebanese levels.

Since the Israeli invasion, Pakradouni said, a "major political evolution" has brought Lebanese Moslems into general agreement with these goals.

Besides disarmament of the Palestinians, Pakradouni said, the Phalangists want an end to "extra-territoriality for the Palestinians in Lebanon." He indicated that the party wants most of the Palestinians to leave the country. Any who remain must be subject entirely to "Lebanese law and sovereignty."