Last week's artillery barrage of Lebanon's Christian heartland by Shiite Moslem and Druze gunners firing from Syrian-controlled areas, apparently intended to beat the Christians into political submission, instead has triggered Christian defiance.

The Christian militias and units of the Lebanese Army responded against Moslem civilian centers with matching firepower and intensity.

"We adhere to our independence like leeches," said Dany Chamoun, the newly elected leader of the Christian National Liberal Party, explaining why the Christian community would not accept the presence of Syrian military observers as sought by the leader of the Shiite Moslem Amal organization, Nabih Berri. "As long as Syria supports Berri and his military solution, I don't see how the Syrians can be neutral observers," Chamoun said.

A cease-fire negotiated by a Syrian intelligence officer with representatives of the Lebanese Army and the Druze, Christian and Shiite militias had called for the positioning of Syrian observers along confrontation lines in and around Beirut. But Berri rejected the Aug. 22 truce agreement on the grounds that stationing the Syrian observers between the combatants was insufficient.

The Shiite Moslem justice minister, whose Amal movement has pounded Christian areas from positions in eastern Lebanon's Bekaa Valley, has insisted that Christian militia and Army artillery encampments be put under Syrian surveillance. His controversial demand has raised questions about how far the Christians are willing to go in accepting Syrian intervention, although Christian leaders have publicly supported Syria's role in Lebanon.

Although Syrian troops did not take part in last week's shelling, the fact that the Syrians looked the other way has fueled suspicions of complicity or an indirect pressure tactic by Damascus.

"The main problem for Christians is the growing Syrian influence in Lebanon," observed a Christian lawyer formerly active in the Lebanese Forces Christian militia. "We paid with a lot of blood, destruction and humiliation to get the Syrians out of some areas," he said. Syrian soldiers pulled out of Lebanon's Christian regions after heavy fighting against the Christian militias in 1978.

"If you let the Syrians in through the window, they will enter by the door," he warned, quoting a current Lebanese proverb. He argued that the presence of a few dozen Syrian observers could become the pretext for bringing back Syrian troops and tanks at the first sign of any friction with Christian residents.

Christian fears of losing more than 40 years of domination brought three aging Christian Maronite ex-presidents -- Camille Chamoun, Charles Helou and Suleiman Franjieh -- together yesterday in a unity meeting in a small Christian village.

For Lebanon's Moslems, political rage is building up against Christian delays in responding to their demands for constitutional changes to provide a more even distribution of power. In the eyes of key Moslem leaders, military solutions and a heavy Syrian presence are the only way to force the Christians to make concessions. The Christians disagree.

"No solution can come from military activity," said Dany Chamoun. "This activity has already achieved the maximum by displacing the population from certain geographical locations. From now on, military activity equals genocide."

The majority of Lebanon's 1 million Christians are concentrated in an enclave stretching north from the center of Beirut and framed from the east by steep mountain ridges. Reflecting growing anxiety within the Christian enclave, Chamoun complained: "The Christians are the most harassed society in the world."

Christian-Moslem tension has set off a rash of kidnapings at the main crossing points between Beirut's Christian and Moslem halves. Civilians living in the capital's formerly secure Christian sections are visibly frightened. Sandbags and bricks block storefronts and entrances to buildings in the residential area just east of the Green Line that splits the city.

"They are really scared, but they have no choice but to be combative and defiant," commented one western diplomat living in the Christian enclave. "They have their backs against the wall. The question many are asking themselves is: 'Shall we just pack one or all of our suitcases?' "

Christian radio stations have been broadcasting upbeat military songs to whip up sagging morale. The launching of the Christian militias' Lebanese Broadcasting Corp. television station last Friday, on the third anniversary of the election of the late president-elect Bashir Gemayel, also was aimed at buoying Christian spirits.

The new TV channel features a daily documentary called "Time of Change," starring the late president-elect and including footage of Gemayel's final days before his assassination Sept. 14, l982.

Gemayel's memory is being skillfully used in a propaganda campaign for Christian confidence-building. Behind this facade, however, serious negotiations are under way among the Christians and with the Syrians. Former president Franjieh, who has always seen himself as an alternative Christian leader to the Gemayel-led Lebanese Forces and Phalangist Party, is now expected to propose a blueprint for constitutional changes based on a 50-50 power-sharing formula between Moslems and Christians.

A Christian source involved in negotiations with the Syrians said they have a major role to play in bringing about reconciliation among Lebanese factions. "Syria could guarantee a consensus among the Lebanese," the source said, adding that Syria's future role would be more political than military. "I don't think you are going to see Syrian observers in Christian areas."

The same source said the Christians were willing to trade some of their traditional prerogatives for real security. However, he pointed out, "reconciliation does not mean that Christians should accept Moslem demands" without exception.

Moslem opposition figures are demanding the abolition of sectarianism in politics, a term referring to the distribution of key government posts according to religious affiliation under a 1943 formula devised by modern Lebanon's founding fathers.

Chamoun echoed these views: "We have to find a system for Lebanon so that all communities can live their lives without fear and participate equally in central government. There must be guarantees for the different communities."

Christian radio stations have been broadcasting ominous predictions of a Moslem assault against Christians, with reports of arms shipments and troop movements in the hills overlooking their tiny enclave. Such gloomy forecasts are making the Christians dig deeper into their bunkers.