BEIRUT, Lebanon (AP) _ Heavy fighting raged today in Christian east Beirut, where Moslem militiamen were said to have intervened in a battle between rival factions of Lebanon's dominant Christian militia.

Police said 25 people were killed and 75 wounded in the Christian sector, and four were killed and 56 injured in Moslem west Beirut.

Artillery shells crashed on both sides of the 3-mile-long Green Line, which divides the Lebanese capital into Christian and Moslem sectors and stretches from Beirut's bomb-ravaged port to the foothills of the central mountains.

Fighter jets of the Lebanese armed forces thundered over the city in low- level runs as battling factions exchanged machine gunfire and rocket- propelled grenades in east Beirut.

Black palls of smoke rose as ambulances and fire engines, their sirens wailing, raced through the Moslem and Christian sectors. Hospitals made repeated radio appeals for blood donations.

Thousands of residents huddled in basements and bomb shelters in east Beirut's Ashrafiyeh neighborhood and the neighboring districts of Karm el- Zeitoun and Saint Joseph.

The fighting among units of the dominant Christian militia, the 8,000- strong Lebanese Forces, began at dawn.

Sources who spoke on condition of anonymity said Moslem militias in west Beirut intervened at mid-morning with a massive artillery barrage against east Beirut.

The fighting appeared to be an attempt by Elie Hobeika, a Christian and the former head of the Lebanese Forces, to wrest control of east Beirut from Samir Gaegea, who ousted him as militia leader in January. Hobeika had signed a controversial Syrian-sponsored peace pact with Moslem militias on Dec. 28.

The Lebanese Forces said in a communique, ''Units from the Amal movement, the Baath Party and various other factions loyal to Syria in west Beirut, including Elie Hobeika, have launched an attack across the Green Line's Sodeco crossing into Ashrafiyeh in east Beirut.''

Amal is the main Shiite Moslem militia, led by Justice Minister Nabih Berri. Baath supports the Syrian government of President Hafez Assad.

If the report of Moslem forces crossing the Green Line is true, it would be their first incursion into the Christian sector since the outbreak of Lebanon's civil war in 1975.

''The attackers were disguised in Lebanese Forces uniforms and are being supported actively by Syrian forces stationed in west Beirut,'' the communique claimed.

''These units have managed to penetrate into Christian neighborhoods across the Green Line. The Lebanese Forces are dealing with this limited incursion,'' the communique said.

Police confirmed the Green Line artillery duels but refused to confirm that Moslem ground forces were fighting in the Christian sector. Witnesses and news photographers said they saw truckloads of uniformed militiamen heading to the Green Line from several Moslem neighborhoods in west Beirut.

Truck-mounted multibarreled rocket launchers and jeep-mounted large caliber recoilless rifles also were seen moving to the central section of the line, witnesses reported.

Christian sources said the fighting centered mainly at the Sodeco and Berjawi crossings from west Beirut to the Christian east side, and around the 25-story Rizk skyscraper in east Beirut's Ashrafiyeh district.

They said Christian units of the Lebanese army were ordered to take control of all Christian areas bordering the Green Line from the Lebanese Forces. The sources, speaking on condition of anonymity, said the buzzing of the air forces' warplanes over Beirut was to provide cover for the army units.

However, police said the planes were sent as a warning to the warring factions to stop shooting or face the army.

Police said today's fighting began between forces loyal to Gaegea and rival groups supporting local Lebanese Forces commanders Maroun Mashalani and Abu Coco. Christian sources said both were loyal to Hobeika.

Mashalani, 26, led an unsuccessful rebellion to oust Geagea in August. Police said he was kidnapped by Geagea's loyalists Friday on a highway north of Beirut.

Sources said leaflets signed by the previously unheard of Christian Republican Army were distributed Friday in Ashrafiyeh. The leaflets accused Geagea and the current leadership of President Amin Gemayel's Phalange Party of seeking to strengthen links between Lebanon's dominant Maronite Catholic community and Israel, the sources said.

Since being ousted by Gaegea in January in fighting that left 350 dead, Hobeika has operated out of headquarters in east Lebanon's Syrian-controll ed Bekaa valley.

The Syrians maintain 25,000 troops in the Bekaa and northern Lebanon under a 1976 peacekeeping mandate from the Arab league.