Beirut — The Lebanese powder keg has finally blown up. This week's fighting in Beirut follows months of anticipation that the war of words between Christian rightists and Muslim leftists would be played out again in the streets of Lebanon's embattled capital city.
But the implications of four days of increasingly vicious clashes - the most serious internal fighting since the civil war of 1975-76 - are as serious for the United States as for Lebanon, for it marks the failure of the year-old policies of both governments. And, although intense negotiations are under way to end the fighting, it will almost certainly require major changes in both US and Lebanese policies to ease the tension permanently.
After an emergency Cabinet meeting Wednesday, President Amin Gemayel invited 11 leftist and opposition figures to join in a national reconciliation conference, including Walid Jumblatt and Amal leader Nabih Berri, as well as rightist Christian figures, including the President's own father.
''Let us rise to the level of the historic responsibility above all consideration and share with the state the decisionmaking to salvage Lebanon,'' he said.
The incident that sparked the warfare was small. On Sunday, Shiite Muslim youths were putting up posters of their missing national religious leader, Imam Musa al-Sadr, in preparation for celebrations of the fifth anniversary of his disappearance after a trip to Libya. Gunmen in a passing car opened fire, killing one and injuring a second Shiite. Shiite and multinational force sources claimed the gunmen were Christian Phalange militiamen.
The attack led Shiite gunmen of ''Amal'' to take to the streets in pursuit, and the Lebanese Army in the area quickly became involved. But the Shiites and other Muslim groups have long charged that the Christian-dominated Army was pro-Phalange, so the Army became a target of retaliation in the poor southern suburbs of Beirut.
By Monday, 13,000 - 40 percent of the Lebanese Army - had to be called in to fight militiamen for what was described by a foreign military adviser as ''the biggest battle the Lebanese Army ever fought.''
Although troops managed to quell the fighting, and a cease-fire was agreed to by noon, it was too late. Other Muslim groups of the Sunni and Druze sects in the capital had by then unearthed weapons hidden for almost a year and joined in as a sign of solidarity.
Battles raged in the streets through Wednesday, as all targets of authority came under fire, including the 5,200-man multinational force from the US, France , Italy, and Britain. Two US marines and five French soldiers were killed in gun battles that saw the US particularly take an active role. The US used Cobra helicopter gunships to take out positions firing at the marines, and brought in the USS Eisenhower aircraft carrier as a show of strength.
But both Lebanese and foreign forces seemed to underline the impotence of power against the determined but vastly outnumbered and outgunned militias, which have long threatened to fight for more representation in a government led and dominated by Maronite Christians.
There has been a raw, almost primitive quality to the fighting, unlike the Israeli-Palestinian conflict last summer when there were usually obvious targets. This time, militias belonging to the Progressive Socialist Party (Druze), Mouribitoun (Sunni), Amal (Shiite), and other smaller groups used rifles and rocket propelled grenades to take on Army tanks and artillery.
They fought from rooftops and street corners, their faces often concealed by pillowcase hoods. Some fought with daring and abandon. During the buildup to the tension last week, one Amal member had predicted: ''We will fight with whatever we have. Life is not worth living without our rights.''
Densely populated west Beirut became pure chaos, with residents in both high-rise apartments and shantytowns taking shelter in basements as the fighting spread. The Monday rush on supermarkets for food and bottled water emptied shelves. By Wednesday, the clashes were so fierce that the government ordered a round-the-clock curfew, which prevented anyone from moving in the commerical and residential areas.
A three-pronged assault by the Lebanese Army early Wednesday finally began to make headway in flushing out the militamen. And Army spokesman captain Youssef Atrissi said, ''God willing, it will be over by the end of the day.''
Yet the problem appears to be far from over. The Gemayel government has still not addressed the underlying discontent and demands that ignited the confrontation. ONE YEAR UNDER THE REAGAN PLAN PLO evacuates Beirut, August-September 1982 Palestinian civilians massacred in Beirut refugee camp by Lebanese militiamen, September 1982 PLO refuses to let Jordan's King Hussein enter US-sponsored peace talks, April 1983 Fighting breaks out between factions of Arafat's Fatah guerrilla group in eastern Lebanon, May 1983 Israel approves 10 new West Bank settlements on Sept. 5, 1982 Israel's Prime Minister Begin rejects Reagan plan, Sept. 2, 1982 US Marines arrive in Beirut Aug. 25, 1982; leave Sept. 10, 1982; return in force Sept. 29, 1982