The Lebanese Army and Christian Phalangist loyalists pushed back a stepped-up offensive by Syria's local militia allies against Christian areas today as a newly bolstered President Amin Gemayel gambled for time following the ouster of his main rival in the Christian camp.

Gemayel's ouster of Lebanese Forces militia leader Elie Hobeika yesterday directly challenged Syria's efforts to strengthen the hand of Lebanon's majority Moslems within the country's complex political order and immediately raised the question of when and how Damascus would respond.

In the Syrian capital, Syrian Vice President Abdul Halim Khaddam continued crisis talks with Moslem militia leaders as the initial Syrian response became clear with the shelling of Gemayel's palace and other Christian strongholds by the Druze and some of the smaller Moslem militias known to have close ties to Damascus.

The sudden upheaval in the country's Christian leadership and the apparent scuttling of a Syrian-brokered agreement once again raised the specter that Lebanon could be thrown into full-scale civil war.

The two key Christian leaders, Gemayel and Lebanese Forces chief of staff Samir Geagea, held talks within their respective ranks, and the Lebanese Forces, the main Christian militia, struck the theme that it is still possible to do business with Damascus.

But the silence from the Syrian capital, accompanied by today's violence, raised questions of whether Syria was of like mind.

As rockets and artillery shells slammed into the Christian heartland and around Gemayel's palace in Bikfaya, the Lebanese Army and Christian loyalists fought back fiercely with U.S.-made M48 tanks, stalling advances on the ground by pro-Syrian militia groups.

Officials of the Lebanese Forces met for the first time since units commanded by the hard-line Geagea linked up with rival Phalangist units loyal to Gemayel to defeat Hobeika, and expressed a desire to remain on good terms with Syria.

A statement said Geagea and his aides sought a "nationalist solution for the Lebanese crisis with the help of Syria." Geagea had opposed the Dec. 28 Syrian-sponsored accord signed by Hobeika and the leaders of the two main Moslem militias, calling it too compromising to Lebanon's Christians.

The Lebanese Forces' statement today called for serious efforts to "put an end to the state of war and establish peace through a new, advanced and open mentality leading to political reforms and setting a more balanced society for all Lebanese."

Gemayel, seeking to strengthen his own position, sent emissaries to former president Suleiman Franjieh, a close Syrian ally but an advocate of safeguarding Christian prerogatives, to ask his support while Lebanese Forces and Phalangist Party officials met in an apparent effort to unify their ranks.

In Damascus, Khaddam met for a second day with Shiite Moslem leader Nabih Berri, Druze chief Walid Jumblatt and members of a broad pro-Syrian coalition of Lebanese leftist and Moslem groups to plan countermoves.

Syria's tactics raised fears of wider operations against Christian areas. Gemayel advisers said today that they doubted Syria would intervene directly, "but nobody is calm, nobody really knows," one said.

A western diplomat noted that any future moves by Syria will have to be done with the "full appreciation of Gemayel's strengthened stature in the Christian community."

Gemayel had raised extensive Christian objections to the accord in his talks with President Hafez Assad earlier this week and had suggested ways to make it more palatable. The accord would apportion more parliamentary seats to Moslems and redistribute some powers of the presidency.

Geagea also has gained from this week's events.

Although the Lebanese Forces' communique signaled a softening, it stressed the "spirit of the March 12 uprising" staged by Geagea against the more moderate Phalangist Party and its Christian militia supporters. The revolt was led to protest overtures toward Syria made by Gemayel with Phalangist backing, and the lack of coordination in decisions concerning the Christian community.

Commenting on this week's events, Hobeika, who fled into exile in Paris yesterday, said today: "Wednesday was a dark day, but anyone who has conquered us will follow us down the same road." Hobeika, who had wrested control of the Lebanese Forces from Geagea last spring, directed negotiations with Syria in contradiction of earlier pledges.

Underscoring the competition for power in Christian ranks, Fuad Abu Nader, a Phalangist military chief, said: "What happened was internal and has nothing to do with the relationship with Syria . . . but if it Syria resorts to force against the Christians, this will unify them."

Forces of the National Syrian Social Party had been mobilized in the hills overlooking the Metn region hours after the siege of Hobeika and his men began Wednesday. Druze militiamen sent reinforcements to the hills southeast of Beirut, maintaining pressure today on the Lebanese Army in the Suq al Gharb axis.

In the north, members of the National Syrian Social Party and some followers of Franjieh pounded Geagea strongholds. Franjieh still harbors a deep grudge against Geagea, who led a militia raid against his summer home in 1978, killing his son Tony and his son's wife, daughter and 30 followers.

Gemayel, who faded into the background in recent months as militia leaders trooped to Damascus to renegotiate Lebanon's political future, could reemerge as the key broker with Damascus now that Syria has lost Hobeika as a guarantor of Christian confidence.