BEIRUT, Lebanon (AP) _ Christian and Moslem militiamen battled with recoiless guns and rocket- propelled grenades in rain-swept Beirut today. Three combatants were killed and 18 fighters and civilians wounded, police said.

Most of the casualties occurred during night-long hostilities in the old commercial district straddling Beirut's shell-shattered port, and along the central part of the city's dividing Green Line, police said.

Fighting tapered off at dawn across the 3-mile line that divides Beirut into Moslem and Christian sectors. The combatants called a cease-fire, but snipers on both sides of the line dueled with silencer-equipped rifles shortly after midday, wounding a woman.

Meanwhile, a cease-fire in the snow-blanketed central mountains above Christian President Amin Gemayel's hometown of Bikfaya was shattered by renewed artillery duels at midday, police said.

The battles pitted army units and Christian irregulars loyal to Gemayel against an alliance of Syrian-backed militias seeking Gemayel's ouster.

Police said they had no immediate casualty report from the new fighting at the Dawwar-Ayroun crossroads near Bikfaya, 10 miles northeast of Beirut. Six people were killed and seven wounded in night-long duels by arillery and tank cannons on the Bifaya front Sunday, police said.

As the fighting flared in Beirut and the mountains, a delegation of Sunni Moslem leaders flew to Damascus for talks with Syrian Vice President Abdul- halim Khaddam.

Grand Mufti Sheik Hassan Khaled, head of Lebanon's economically powerful but militarily weak Sunni community, told reporters as the talks began, ''We are discussing the situation in Lebanon.''

A Syrian-brokered peace pact signed Dec. 28 and aimed at ending Lebanon's 11-year-old war collapsed in January triggering more fighting in which police said 350 people died.

Syria, Lebanon's main power broker, has about 25,000 troops stationed in north and east Lebanon under a 1976 peace-keeping mandate from the 21-nation Arab League.

In a separate development, tensions were reported high in the Syrian- controll ed north Lebanon port city of Tripoli today following the assassination Sunday of Khalil Akkawi, 43, a prominent anti-Syrian fundamentalist Moslem leader.

Armored Syrian units patrolled the Bab Tabbaneh district where a funeral was scheduled for Akkawi, a leader of Tawheed Islami, or the Islamic Unification Movement.

He was killed in a hail of automatic gunfire as he drove through the outskirts of Lebanon's second largest city.