BEIRUT, Lebanon (AP) _ Shiite Moslem leader Nabih Berri, bent on ousting Christian Amin Gemayel from the Lebanese presidency, said Monday that Syria will sponsor a meeting next month to try to unite Lebanon's feuding Moslem factions.

In central Beirut, gunbattles flared anew between Christians and Moslems, and between Lebanese and Palestinians.

One combatant was killed late Sunday and 10 others were wounded in clashes between Berri's Shiite Amal militia and Palestinian guerrillas in the Chatilla refugee camp.

Christian and Moslem militiamen hurled grenades at each other across the Green Line that bisects the city into Moslem and Christian sectors, sparking a three-hour mortar duel, a police statement said. It said two civilians were killed and six others wounded.

In May and June, the Shiites and Palestinians had fought for five weeks for control of the Beirut refugee camps. Syria arranged a cease-fire June 18, but it has since been violated frequently. Christian-Moslem fighting along the dividing line also has escalated in the past few days.

Syria, main power broker in Lebanon, has sought to resolve intra-Moslem feuds in preparation for an overall settlement of Lebanon's decade-old civil war. Last week, Damascus sent six military officers here to help enforce a security plan to end lawlessness in the western, Moslem half of the capital.

The plan called for an end to militia control of west Beirut and is supposed to be extended eventually to the city's mostly Christian eastern half. But so far it has failed to bring the various militia factions to totally withdraw their gunmen from the streets.

Berri, who arrived from the Syrian capital of Damascus over the weekend after a two-week visit with his Syrian allies, said a meeting of Lebanon's Islamic groups would be held in early August in east Lebanon's Syrian- controlled town of Chtaura.

The Gemayel government ''must be changed because it is the regime that invited Israel into our country,'' Berri said, referring to the 1982 Israeli invasion of Lebanon, during which the Gemayel family's Phalange Party militia cooperated with the Israelis.

''The regime must be replaced by one to which all Lebanese become loyal,'' Berri said.

The militias of both Berri and his civil war ally, Druse leader Walid Jumblatt, have battled the Gemayel government's Lebanese army in recent violence.

A Syrian-sponsored Lebanese reconciliation conference in Switzerland in March 1984 produced a cease-fire and a national coalition government of which both Berri and Jumblatt are members.

Berri, who serves as justice minister and state minister for south Lebanon, also condemned Israel's attack Sunday on two Shiite villages in southern Lebanon. He vowed to escalate guerrilla warfare against the Israelis.

''Israel hasn't learned yet that such operations only increase our determination to continue the struggle until total liberation of our land,'' Berri said.

At least three Lebanese were reported killed in the Israeli raids on the southern Lebanese villages of Qabrikha and Sejoud.