President-elect Bashir Gemayel is counting on a dramatically expanded national armed force to impose his rule on divided Lebanon when he takes office later this month.

Gemayel's advisers said today they hope the U.S. government will provide the aid, in money as well as arms, to shape this modern force.

But these close advisers to the 34-year-old Phalangist Party leader said that until this new Army is in control of the country, his own Christian militia, the Lebanese Forces, will not be disbanded as demanded by nervous Moslem leaders who have fought against it since the 1975-76 Lebanese civil war.

Among Gemayel's first acts when he takes office Sept. 23, these advisers said, will be to order the untested Lebanese Army to take full control of Moslem West Beirut, to date still in the hands of a motley collection of leftist and Moslem militia groups despite the recent introduction of national police for the first time since the civil war.

The introduction of the Army into West Beirut is to be done simultaneously with a government order for the immediate doubling of the 23,000-man Army which has only recently been constituted after falling apart during the civil war. This expansion, to be accomplished through obligatory national service is to be the first phase of an expansion that Gemayel's advisers say they hope will result in a strong "army of war and defense" of 100,000 men by 1984.

The advisers say it will have to include a navy to patrol the long coastline as well as a modern air force to control the skies that have been contested so frequently by the Israeli and Syrian air forces.

According to these sources, the question of U.S. aid was raised by Gemayel last week in a meeting with visiting U.S. Secretary of Defense Caspar W. Weinberger who promised that all of his requests would be given "serious consideration." Gemayel's aides would give no further details of Lebanon's military aid requests.

Putting the Lebanese Army in charge of security in West Beirut could provide the new president with his first test of authority. Following the evacuation of the Palestine Liberation Organization that for the past decade has been a dominant force in West Beirut, the city's Moslem leaders have refused to allow the Army to assume any security tasks in their sector because they continue to suspect it of being a tool of the country's Christians. Its higher commands are dominated by Christian officers even though there are many Moslem soldiers. Gemayel's militia has also had conflicts with the Lebanese Army, at one point undermining an Army effort to protect a rival Christian militia.

The Moslem leaders in West Beirut so far have only agreed to the introduction into the sector of national policemen, limiting the Army to the defense of government buildings and the radio and television stations and their traditional, now mostly bombed-out, barracks.

Under a compromise agreement for reestablishing government authority in West Beirut worked out last week between the city's Moslem leaders and Prime Minister Shafiq Wazzan, also a Moslem, the Army was only to be used as a backup force to help the police in an emergency--and then, only with the approval of the prime minister.

Gemayel's aide said, however, that Gemayel would insist that the Army take over security responsibilities in Beirut, not the national police.

The Army, however, has been allowed to take up positions previously held by the PLO and its various Lebanese leftist militia allies in order to separate the city center from the Israeli Army.

The Army finally completed this task today when it took up positions in the southern outskirts of West Beirut, just across from the Bir Hasan district into which Israeli forces advanced last Friday.

The Army move into the Jnah district, just north of Bir Hasan, was arranged hurriedly last night after Israeli Defense Minister Ariel Sharon declared during a visit to his troops in the area that if the militias facing them did not pull back the Israeli Army would advance farther into West Beirut. Gen. Sharon said he had no intention of ordering a withdrawal of his forces until he was satisfied that the Lebanese Army had established its control of West Beirut.

Wazzan has asked U.S. officials to use their influence to gain an Israeli withdrawal from around the city now that the PLO fighters have been evacuated in accordance with the agreement negotiated by U.S. special envoy Philip C. Habib.

Israel has indicated it will not withdraw from its positions around the city until the hodgepodge of Lebanese militia groups, ranging from the highly disciplined Shiite Amal to half a dozen Nasserite factions, give up the heavy weapons they received from the PLO in contravention of the Habib plan. The accord had envisioned these weapons being surrendered to the Lebanese Army.

The question of the militias, both Christian and Moslem, is one of the major issues still dividing Gemayel, a Christian Maronite, and the Beirut Moslems who vehemently, but unsuccessfully, opposed his election by parliament last month.

The Army originally broke down during the civil war when Moslem officers bolted to fight on the side of the Moslems and the PLO, while Christian officers bolted to join the ranks of the Christians whose dominant militia was under the command of Gemayel, the son of the rightist Phalangist Party founder, Pierre Gemayel.

In 1978, two years after the civil war had been halted by the arrival of an Arab League peace-keeping force, mostly from the Syrian Army, the Lebanese Army was reconstituted. But it was never allowed to control much more than several dozen square miles around the presidential palace in Baabda, east of Beirut.

In 1980 it was finally ordered into action in the East Beirut suburb of Ain Rumaneh to defend a group of Christian militiamen of former president Camille Chamoun's National Liberal Party threatened by Gemayel's Lebanese Forces, who were bent on eliminating any Christian rivals.

The Lebanese Army bowed to the superior force of Gemayel's militia, allowing it to attack and kill the members of Chamoun's militia group.