The Lebanese Army deployed troops in East Beirut today, taking control of the Christian sector of the capital for the first time in eight years.

The move of government troops into East Beirut before dawn followed months of intense negotiations between the government of Lebanese President Amin Gemayel and the Christian militiamen who had ruled over the area since the Army splintered along sectarian lines during the 1975-76 civil war.

Gemayel, dressed in a military uniform in his role as supreme commander of the armed forces, personally gave the order to the troops to fan out to East Beirut and the Christian suburbs.

Flanked by his senior military officers, who were given sweeping powers to maintain law and order under the government decree for the deployment, Gemayel spoke for one hour explaining the objectives of the new move.

"We have the responsibility of saving Lebanon and liberating it," Gemayel said in the speech to the troops, which was later broadcast on radio. "The eyes of the world are on us. Our friends expect us to succeed, while others are lying in wait for us. Your duty is to succeed. This is the hour of pride, and the world has been waiting for this step. Be up to the mission, and let citizens feel the existence of the state."

"We want to succeed in unifying the capital," Gemayel said. "We want to succeed in unifying the hearts of the Lebanese."

By mid-morning, the jeeps, tents and armored personnel carriers of the 4,000 Lebanese Army soldiers were spread out over Christian neighborhoods and suburbs.

The satellite barracks of the Christian Lebanese Forces militia were vacant, the streets empty of the militiamen who are now forbidden to appear in East Beirut in military vehicles, wearing uniforms or carrying weapons.

Under the agreement Gemayel reached with the militia, which was created by his slain brother Bashir, they were permitted to maintain their headquarters in East Beirut, and they still retain control of the busiest part of the port of Beirut.

Even as the fledgling Gemayel government was extending its control over the capital, fears were raised anew that occupation by Israeli, Syrian and Palestinian soldiers in virtually all of the rest of the country would lead eventually to a permanent carving up and cantonization of Lebanon.

The fears were raised by the declaration yesterday by renegade Lebanese Army Major Saad Haddad that his Israeli-equipped forces had taken control of a 25-mile zone of southern Lebanon that Israel now occupies and has said it wants established as a "security zone."

Haddad's declaration and his establishment of a garrison post in Sidon, Lebanon's southern provincial capital, were not regarded as being serious in themselves here because of the scant size of his force and his unpopularity in the south.

Haddad moved today to expand his control of south Lebanon, placing a garrison in another town and announcing he had dispatched a unit to the Bekaa Valley, United Press International reported.

Witnesses reported that Haddad, accompanied by Israeli officers, paraded through the major market town of Nabatiyeh, 20 miles southeast of Sidon, with tanks and other armored vehicles.

The declaration and the parade by Haddad through the streets of Sidon yesterday were scrutinized here for leads on Israeli intentions in southern Lebanon.

There was some suspicion here that the announcement may be linked in some as yet unknown plan by Israel to retain a permanent presence in Lebanon. Western diplomatic sources said tanks that Haddad paraded through the streets of Sidon had been brought up from Israel the day before. They also noted that the display came on a day when Israeli negotiators were being especially tough and uniyielding in talks aimed at achieving the withdrawal of Israeli soldiers.

Israeli troops shot at a civilian car today, killing three of its occupants and wounding a fourth in a residential neighborhood east of Beirut, the Associated Press reported, quoting police sources.

The Israeli command in Tel Aviv said the car carried guerrillas who opened fire on an Israeli Army roadblock at Monteverde, six miles east of Beirut. It said the Israelis returned fire.

The Lebanese Army units that went into East Beirut today had undergone special training under the supervision of U.S. Marines in the multinational force here.

Diplomatic sources said the ambassadors of the four countries contributing to the multinational force--the United States, France, Italy and Great Britain--were meeting to decide whether and to what extent their troops would follow the Lebanese Army into East Beirut.

The 4,500-man multinational force is currently deployed almost entirely in West Beirut and the suburbs just south of that predominantly Moslem sector.

President Gemayel had moved quickly to disarm the more than 40 militias operating in West Beirut and establish the Army's presence there last September.

His failure to do the same in East Beirut had created resentments among Moslems, who began to tauntingly refer to Gemayel as the "president of West Beirut."

A story emerging now from Gemayel intimates and diplomatic sources is that his hesitation was based not only on the intransigence of the Lebanese Forces Christian militia but also on Gemayel's concern that they might attempt a coup against his government. Today, the Voice of Free Lebanon, the Lebanese Forces radio station, welcomed the arrival of the Lebanese Army to "our area." It "has proven that the system of the Lebanese Forces has been at the service of building a nation and building a state."