A major showdown between Christian militias involving tanks and ferocious combat dealt a severe blow to a Syrian-brokered peace pact today and plunged Lebanon into a new cycle of violence.

Eli Hobeika, commander of the Lebanese Forces, the main Christian militia, and his followers were crushed in a strike by his own chief of staff, Samir Geagea and Phalangist fighters loyal to President Amin Gemayel.

Hobeika, who had challenged the Maronite Catholic president for leadership of Lebanon's 1.5 million Christians, surrendered to a Lebanese Army commander. He and his supporters were escorted out of their besieged stronghold by the Lebanese Army, late evening reports said.On Monday Hobeika had launched an assault on Phalangist positions at access routes to the mountainous Metn region, a Gemayel stronghold.

Pro-Syrian militias entrenched in the hills overlooking the Metn started pounding Gemayel's hometown of Bikfaya at midafternoon after Hobeika had been encircled in his bunker with his men.

After seven hours of tank and artillery duels and hand-to-hand fighting in which Geagea's supporters tightened the grip around the Lebanese Forces command center and were in turn besieged by reinforcements coming to Hobeika, he lost control of three radio and one television station.

The Gemayel-Geagea forces were reported in control of east Beirut and most of the Christian heartland by nightfall. Fighting had diminished to sporadic gunfire and explosions.Today's battles splintered the executive committee of the Lebanese Forces, which had staged an uprising in March against the established leadership of the Phalangist Party, considered too supportive of the president.

Gemayel has doggedly refused to go along with the Syrian-engineered peace agreement, signed last month by Hobeika, Druze leader Walid Jumblatt and Shiite Moslem leader Nabih Berri.

His resistance to the accord won him the support of Christian traditionalist leaders and rival Christian militia strongmen such as Geagea, who strongly object to Syrian dominance in Lebanon and what they consider drastic political concessions to the Moslem majority.

There was no official word from Damascus, where Gemayel had cited Christian rejection of key clauses of the agreement as grounds for revising it during two days of summit talks with Syrian President Hafez Assad.

Druze militia deployment in the hills overlooking Beirut and the presidential palace at Baabda and advances by other pro-Syrian groups along the coast toward Geagea's fortifications south of the Madfoun bridge, 25 miles north of Beirut, raised fears of a Syrian-backed offensive.

Political sources said Palestinian gunners, dug into hilltop positions overlooking the Metn and Christian areas, may be participating in the shelling.

The Lebanese Army issued a statement saying that three Lebanese soldiers were killed in an attack by "armed elements" against the Douar position northeast of Bikfaya, seat of the Gemayel family and the summer presidential palace.

No exact casualty toll could be compiled for today's fighting, but several radio stations said at least 100 persons were killed.

Scores of houses, factories and fuel tanks were set ablaze by rockets, mortars and artillery shells fired into the combat zones. Bishop Ibrahim Helou, acting head of the Maronite Church, appealed to the combatants to spare civilians trapped in hospitals and schools.