A serious split within Lebanon's Christian community worsened today, posing new threats to the country's remaining stability and a major challenge to the government of President Amin Gemayel.

Samir Geagea, a top commander of the Lebanese Forces, the combined Christian militia, declared himself and his forces in revolt from the Gemayel government. Geagea's supporters, who are allied with Israel, have accused the Gemayel government of being too close to Syria.

Militiamen allied with Geagea took control of parts of Christian east Beirut, and at least two important barracks of the Lebanese Forces in Christian territory north of Beirut.

Gemayel, who heads the Christian Phalangist Party, canceled a trip to Moscow for the funeral of Konstantin Chernenko and convened Christian leaders here in efforts to find a solution, as did Cardinal Antonius Boutros Khreish, patriarch of the Maronite Catholic Church.

Hostilities continued in southern Lebanon, between Israeli occupation forces and Shiite Moslem residents of the area, with at least one outbreak of shelling reported but no casualties.

Meanwhile, Israeli warplanes attacked a building in Bar Elias, in the Bekaa Valley, that the Israeli Army command in Tel Aviv said was headquarters for a unit of the pro-Syrian Saiqa Palestinian militia. Syria said the Israeli planes were driven off but Israeli officials said they scored "accurate hits." There were no reports of casualties.

The appearance of Lebanese Forces and Phalange supporters in different uniforms in east Beirut and north of it today spread tensions. Shops and schools closed in fear of an armed showdown that did not materialize.

Geagea (pronounced zha-zha), who led the break with Gemayel, controls the Christian militias' artillery, antitank and mechanized units and is in charge of the Lebanese Forces' mobilization program, giving him broad leadership powers.

His revolt was prompted by his expulsion yesterday from the Phalangist Party after he refused to dismantle a checkpoint at the Madfoun Bridge at Barbarah on the Beirut-Tripoli coastal highway north of Beirut. The checkpoint brought lucrative truck tolls to the Lebanese Forces, but Syria, whose troops are in the Tripoli area, wanted it removed as part of a broader security plan.

Geagea, known for his uncompromising brand of politics, was joined by the Christian militias' intelligence chief, Elie Hobeika, according to aides. Hobeika, closely allied to Israel, has been accused of involvement in the massacres at two Palestinian refugee camps in Beirut in September 1982.

A firm believer in Christian supremacy, Geagea has accused the Phalange Party of distancing itself from the rank and file of the Christian forces and of losing sight of its main objective, the guarantee of freedom and protection to Lebanon's Christian community.

Yesterday, Geagea's supporters took control of Lebanese Forces barracks in the Jubayl and Kesrouan regions north of Beirut with no resistance and of the Voice of Free Lebanon, the radio of the Lebanese Forces.

Gemayel called 63 Christian political and religious leaders to the presidential palace in Baabda today for emergency talks to defuse the militia rebellion and bring it under control. Phalange troops loyal to Amin cut off the coastal road north of Beirut at Dbaye to prevent a spillover into the capital from the north and went on alert.

The commander of the Lebanese Forces, Fuad Abu Nader, a nephew of Gemayel, met with other senior commanders in a bid to bring Geagea and his followers back under his wing. Abu Nader, sensing discontent among Christian fighters, issued a statement at 2:30 a.m. today saying that the Lebanese Forces had recovered their independent will and freedom in decision-making.

Another statement issued by the forces' central command later today said the militia was not an instrument for coups.

It was not clear where Abu Nader stood, and spokesmen for Geagea said tonight that the Lebanese Forces commander would have to choose.

A Lebanese Forces spokesman, Elie Khayyat, who uses the code name of Tareq, said the style in which Geagea had expressed himself was wrong but there is general sympathy with his ideas.

Tareq cited Syrian pressure last year that led to the abrogation by Gemayel of the 1983 Lebanese-Israeli troop-withdrawal agreement and alleged Syrian intervention in internal Lebanese affairs.

He said it was appalling that Syrian Vice President Abdel Halim Khaddam would sit with Lebanese Cabinet ministers as they decided major issues -- as Khaddam did last week. Tareq criticized Syria for pressuring Lebanon into canceling decrees passed by the former government.