Fighting broke out along the city's dividing Green Line Thursday after Parliament failed to elect a new president, and state radio said gunmen killed two senior commanders of the mainstream Shiite Moslem Amal militia.

Parliament sources said President Amin Gemayel would announce a caretaker Cabinet later Thursday led by a Christian as prime minister in a move that could spark renewed conflict between Moslems and Christians.The two groups have been fighting a civil war for the past 13 years, ravaging the once prosperous economy of this nation and dividing the country into factions controlled by bands of guerrillas.

Daoud Daoud and Mahmoud Fakih, top Amal officers for southern Lebanon, were killed instantly when a rocket-propelled grenade hit the car they were riding in on the Ouzai highway, the broadcast said. The highway is in Syrian-policed south Beirut.

Daoud and Fakih, whose Amal Shiite movement is backed by Syria, led a crackdown on pro-Iranian Shiite fundamentalists in south Lebanon earlier this year. That operation ousted the extremist Hezbollah, or Party of God, from most of the southern villages.

Shell explosions and bursts of gunfire echoed across Beirut after the Parliament speaker Hussein Husseini, a Shiite Moslem, put off voting for 24 hours in a last-ditch effort to avert a showdown that is threatening Lebanon with partition.

Gemayel must step down at the end of his term, which by tradition ends Friday.

Police said one Christian militiaman was killed and two soldiers of the regular army's predominantly Shiite Moslem 6th Brigade were wounded in mortar and machine gun duels near the old Parliament headquarters.

Under an unwritten national covenant dating from independence in 1943, the Lebanese president must be a Maronite Christian, the prime minister a Sunni Moslem and the Parliament speaker a Shiite Moslem.

Moslems have said that if Gemayel appoints a Christian to be prime minister, they will declare their own republic in territory they control. That would formalize the breakup of Lebanon into rival sectarian mini-states.

Former President Charles Helou said after meeting with Gemayel the new prime minister would probably be "a member of the Helou clan." It was an apparent reference to his cousin, Pierre Helou, a Maronite Catholic businessman who maintains close ties with several Moslem leaders.

Three Christians are standing for election to replace the Maronite president. Syria backs two of them: former President Suleiman Franjieh, 78, and 58-year-old Parliament Deputy Mikhail Daher.

The third, veteran politician Raymond Edde, has been living in self-imposed exile in Paris for 12 years after escaping three assassination attempts in Lebanon.

Edde, 75, is the most popular in public opinion polls. But Daher's chances appear stronger in Parliament because he is backed by the United States as well as Syria, Lebanon's main power broker. Damascus has 40,000 troops in the country.