Lebanon's Shiite Muslims flex their military muscles

Beirut's Shiite-Muslim neighborhood begins where the posters of Arab socialist hero Gamal Abdel Nasser, Palestinian hero Yasser Arafat, and Druze hero Kamal Jumblatt end.

All over the once-fashionable French colonial buildings of Patriarcat and Basta are posters of Musa Sadr, the Lebanese Shiite spiritual leader who mysteriously disappeared on a visit to Libya in August 1978.

These neighborhoods of pitted streets and plywood and cardboard sidewalk stalls - poorer than most in Beirut - were the scene of fierce fighting last week.

The gunfire in Patriarcat and Basta began Jan. 6, the Orthodox Christmas. At first, the shooting did not seem unusual in the mixed Muslim-Christian city. Most celebrations here - from a wedding to the assassination of Anwar Sadat - are marked by gunfire.

But as the evening wore one, the rattle of small-arms-fire gave way to the sobering boom of mortars and rocket-propelled grenades. Yellow patches of fire lit up the night sky from the now-off-limits neighborhoods.

These are the violent growing pains of the increasingly important Shiite element in Lebanon.

No one is quite sure what touched off the Shiite warfare. Three current theories, in order of popularity, are: refusal of a shopowner to pin up a photo of a Shiite militiaman ''martyr;'' an argument by two Shiite brothers over politics; and a gangland dispute over vice trafficking.

Soon, however, the fight broke down into a major street war between the two most powerful Shiite militias, Amal (struggle) and the Organization of Communist Action in Lebanon (OCAL). The fighting went on for three days until it was stemmed by a cease-fire and intervention of Syrian troops and their Lebanese leftist allies. Seventeen persons were killed, 40 injured.

Whatever the spark that set it off, Shiite tension had been building in recent months. Diplomats in Beirut say that - in the usual way parties form and grow in the anarchy of Lebanon - the Shiites, specifically those of Amal, are flexing their military muscle. They are making themselves a force to be reckoned with in any future ''national reconciliation'' in the Lebanese civil war.

What appears to be happening, say these diplomats, is that the Shiites of Amal are growing strong enough militarily that they are attempting to eliminate rivals such as OCAL and stake out territory.

This kind of pattern was established in the l960s and '70s by the Palestinians, the Maronite Christian Phalangists, and the various Arab socialist groups. Each now has its own turf - hardly distinguishable from other parts of the city on most days. But when fighting starts, the neighborhoods become enclaves, the buildings turn into sniper perches.

The significance of the growing strength of Amal is threefold:

* It represents the increasing militancy of Lebanese Shiites, who are believed to constitute a majority among Lebanese Muslims, though no census has been taken since the civil war began.

* It indicates the consequences of large-scale uprooting that the Israeli-Palestinian conflict in southern Lebanon has had on the largely Shiite peasantry.

* It reflects an Iranian and Syrian effort (the two countries are believed to be selling weapons to Amal) to create a non-Sunni, non-Christian, non-Palestinian force in Lebanon. The Shiites of Iran and the Alawites who rule in Syria are close to each other religiously. In strengthening Amal, Syria's own interests are served in that Amal keeps the PLO occupied.

The Shiite awakening in Lebanon goes back to a Shiite general strike that Musa Sadr called in 1970 in order to draw government attention to the problems in southern Lebanon and the social inequality he felt the Shiites were experiencing.

Musa Sadr's popularity and the strength of his ''movement of the underprivileged'' grew throughout the 1970s. His disappearance in 1978 caused a rift between the Shiites and Libya's Muammar Qaddafi.

Just last month, Shiite activists tried to bring Sadr back: A jet bound for Libya was hijacked by a group of Lebanese Shiites demanding Sadr's return.