The Lebanese Army, taking on a significant new responsibility, has replaced Israel as peace-keeper in a mountain village torn by Christian-Moslem fighting, officials said today.

The Israelis insisted on leaving two tanks and a squad of soldiers on the edge of the village to ensure that the Lebanese could handle the job, an Israeli military spokesman said. In several days of arguments over how the changeover should take place, Israel apparently was reluctant to yield authority in territory now occupied by Israeli troops.

Beirut newspapers reported that U.S. diplomats here pressured Israeli authorities to cooperate in allowing the Lebanese Army to move in. The United States wishes to build up the Army to help restore central government authority, challenged since the 1975-1976 civil war by an array of militias representing religious and political groups.

Correspondents for Lebanese state television accompanying President Amin Gemayel reported that the United States has agreed to supply Lebanon soon with enough weapons to equip three brigades, about 7,500 men.

Gemayel returned to Beirut today from a five-day trip to the United States, France and Italy. The television report also said France is to give Lebanon about $85 million worth of military hardware.

In one sign that the Army is gaining respect, about 300 residents of West Beirut staged a spontaneous demonstration yesterday to protest what they feared to be the Army's withdrawal from their neighborhood. The residents of the predominantly Moslem section of the city apaparently were afraid that they would be left without protection against Christian militiamen, who massacred hundreds of Palestinians in camps last month, or against criminals and lawlessness.

The protesters dispersed when an Army officer told them the troops' movements were a routine rotation.

The Lebanese Army sent its first soldiers into the mountain village of Kfar Matta, about 15 miles southeast of the capital, on Monday. It gradually reinforced the unit with armored personnel carriers and more than 100 troops, and Israel formally yielded responsibility for policing the village yesterday.

The Israelis posted troops and tanks in Kfar Matta and several nearby villages last Friday to halt four days of clashes between Moslem Druze and Christian Phalangist militiamen in which five persons reportedly were killed.

Senior Lebanese government officials have accused the Israelis of inciting such fights to justify the need for Israeli troops to remain to maintain order. Former prime minister Saeb Salam was quoted by the London-based magazine Al Hawadess as saying in an interview that Israeli Defense Minister Ariel Sharon was arming both Phalangists and Druze to promote sectarian tension in Lebanon.

The Israelis moved into the mountains to stop the fighting after the Druze community in Israel protested to the government there that Lebanese Druze were in danger. The Druze faith is a mystical offshoot of Islam believing in reincarnation, and its followers have clashed with Christians in Lebanon since the last century.

President Gemayel presided over a meeting of Phalangist and Druze leaders last Sunday. It was agreed in principle that the Lebanese Army ultimately would take over peace-keeping duties from Israel.

The Lebanese did not wish to send in troops until after the Israelis had withdrawn, reportedly feeling that Lebanese sovereignty would be infringed if Lebanese soldiers had to share duties with the Israelis. Israel, however, demanded an overlapping period of two days or more between Lebanese deployment and Israeli withdrawal.

The two sides finally worked out a compromise in which the Lebanese took over full police duties inside Kfar Matta but the Israelis stayed nearby for an unspecified period of time, the Israeli spokesman said by telephone. He said the Lebanese had stationed 200 soldiers in the town.

Beirut state-owned radio said that the Lebanese Army also had taken positions in two other villages in the area. The Israeli spokesman denied the report, however, and said in any case the Israelis had not given up responsibility for any village except Kfar Matta.

The Israeli spokesman suggested the Lebanese should disarm the Druze to stop the fighting. It was unclear why he did not expect the Army also to disarm the Phalangists, except that the Phalangists and Israelis are informally allied.

In a separate development, both Syria and Palestinian leader Yasser Arafat suggested that an outbreak of fighting was likely soon in the Bekaa Valley in eastern Lebanon. Damascus radio, in an item that the topped its newscast, accused Israel of reinforcing its troops in the Bekaa and preparing for an attack there.