Rival Christian militiamen battled today for control of strategic routes north and east of the capital as President Amin Gemayel flew to Damascus for critical talks on the fate of a Syrian-sponsored reform package aimed at ending the country's 11-year-old civil war.

At least 12 persons were killed and dozens wounded in the fighting, the worst in Christian areas this year.

At daybreak, the Lebanese Forces, the main Christian militia, launched a multipronged attack against Phalangist rivals that involved several districts of east Beirut. Banks and schools closed, and thousands of students and office workers were stranded by the fighting.

Jeeps full of militiamen poured onto the coastal highway north of the capital and into suburban areas. Fuel tanks set ablaze north of Beirut were still giving off clouds of black smoke at sunset.

After daylong clashes involving mortars, rocket-propelled grenades, recoilless rifles and machine guns, the Lebanese Forces isolated the hilly Maten Province in the Christian hinterland, Gemayel's stronghold, from his presidential palace in Baabda, militia sources said.

Public disagreement over the Damascus peace accord, signed by the commanders of the Lebanese Forces and the country's two largest Moslem militias on Dec. 28, has highlighted a power struggle within Christian ranks.

Traditionalist Christian leaders, reluctant to hand over power and privileges to the Moslem majority, have balked at the accord, which calls for phased reforms curbing the authority and prerogatives of the president, wider Moslem representtion in parliament and a virtual grounding of the Lebanese Army pending an unspecified period under Syrian supervision.

Gemayel was planning to convey this opposition during his summit meeting with Syrian President Hafez Assad in hopes of discrediting his Christian militia foe, Elie Hobeika, one of the signatories.

The escalation in inter-Christian hostilities and fighting appeared to undermine Gemayel's bargaining position in the Syrian capital. Assad did not meet him at the airport today as he has for previous summits.

Hobeika charged today in a long communique justifying the fighting that Gemayel supporters had launched a campaign of intimidation against his followers in the Maten. Hobeika, a harsh critic of Gemayel, announced this morning that units of the Lebanese Forces had been given orders to "chase agitators" from the area and "deter them from troublemaking. . . . "

He lashed out against "mercenaries working for the master of the palace [Gemayel]," blaming them for a series of kidnapings and other actions against his men.

Sources close to Hobeika said that today's operation was intended as a warning to Gemayel to "adjust his political stance to the military balance on the ground."

Internal bickering in the leadership of the Lebanese Forces appeared to have weakened Hobeika's political standing in the Christian community. Hard-liners such as senior Christian commander Samir Geagea object strongly to the Syrian-brokered peace accord because they believe it would formalize Syrian tutelage over Lebanon.

Geagea, who commands the armor and artillery units of the Lebanese Forces as well as their tank divisions, stayed out of the fighting today.

Following a failed assassination attempt against one of his deputies on Dec. 31, in which Hobeika believed that he was the intended target, the Lebanese Forces commander blamed Gemayel.

Phalangist militiamen from the Maten, still loyal to Gemayel, tried to take advantage of Lebanese Forces' conflicts to undermine Hobeika.

But despite recent setbacks to Hobeika's grip on the Christian community, he still controls the bulk of the Lebanese Forces, which are estimated to number 6,000 men.

"For a while, Hobeika seemed like a general with no soldiers; now he is trying to prove he does have soldiers," said a U.S. diplomat in east Beirut who is closely monitoring the Christian power struggle.