he Lebanese Army finally moved into the center of Christian East Beirut today with the support of the French and Italian peace-keeping forces -- but not the U.S. Marines. The Marines were supposed to join in the operation but failed to do so, apparently because Washington had not sent the final order for their deployment.

After several delays, units of the Lebanese Army established positions and checkpoints at five points in residential areas without meeting any resistance from the Christian Lebanese Forces militia which has ruled the Christian sector of the capital ever since the 1975-76 civil war.

Lebanese Forces leaders had agreed in negotiations with the government to keep their troops off the street and confined to barracks. A few armed militiamen were seen today but they caused no trouble.

At least one French patrol consisting of three jeeps filled with paratroops moved through the streets, while a spokesman for the Italian contingent of the multinational peace-keeping force said it would begin sending in similar patrols starting as 10 p.m.

Twelve U.S. Marines in four jeeps waited all day at their headquarters compound outside the international airport for orders to join the French and Italian patrols. But the orders never came.

Capt. Dale Dye, a headquarters spokesman, said the Marines had not gone in because we "did not receive an executive order from our higher headquarters. We are on hold and will continue to hold."

Chris Ross, a State Department official here with U.S. special envoy Morris Draper, attributed the delay to "procedures" and said it was "just a matter of getting the orders passed down the chain of command."

One report circulating here, however, said there was still some slight disagreement among the three Western governments about where each country's force should go inside the eastern sector.

The American troops are now expected to begin patrolling alongside their French and Italian counterparts Thursday.

French troops first began deploying around the Christian sector in mid-October as the first Lebanese Army units took up positions on the fringes. But neither the French nor the Lebanese had moved into the downtown area before today.

The Army faced a delicate political and military problem in entering East Beirut because the Lebanese Forces militia has a total strength of roughly 20,000 regular and reservist troops, nearly as large as the entire regular Army.

But the entry was made after Christian militia leaders had already moved most of their heavy arms out of the city and pledged to cooperate.

The two sides also appeared to have reached a compromise whereby the kind of house-to-house search for arms and aliens that took place in Moslem West Beirut last month would not be conducted on the Christian side.

During the sweep of West Beirut the Army picked up 1,440 Palestinians, foreigners and Lebanese and held nearly 1,000 of them on various charges ranging from criminal offenses to invalid residence permits. It also uncovered tons of arms and other war materiel belonging to the departed Palestinian guerrillas and other leftist Moslem militias.

Meanwhile, the situation in the mountainous Chouf region southeast of the capital was reported today still extremely "tense" following more clashes Tuesday and last night between Christian and Moslem Druze militiamen.

The Christian radio Voice of Lebanon this morning reported 14 more persons had died overnight in scattered incidents while the leftist As Safir newspaper said 20 had died and "tens of others" were wounded. The official government count put the deaths at eight.

The two villages of Brih and Kfar Nabrakh, the first mainly Christian and the latter Druze, were surrounded today by Israeli troops seeking to stop the fighting which has been going on sporadically since last month, Beirut radio said.

The two warring villages, separated by a valley, are located south of the Beirut-to-Damascus highway near Ain Zhalta deep in the Chouf mountains. They were the scene of intense strife back in 1977 after the Druze leader Kamal Jumblatt was assassinated.

President Amin Gemayel spent much of the day huddled with officials discussing ways to halt the sectarian fighting, which has been spreading to other areas of the Chouf.

Prime Minister Shafiq Wazzan said the government had decided on "new measures" to stop what Beirut radio described as "the deterioration in the security situation" in the Chouf. Wazzan did not disclose what the measures were but he said the situation was "very serious and could have great consequences."