At 46, tall, soft-spoken Nabih Berri, the leader of the mainstream Amal movement, has emerged as the chief spokesman for growing Shiite Moslem power in Lebanon.

Skillfully riding the turbulence of Shiite fervor and seizing opportunities, Berri has managed to forge a link between religious radicalism and political demands.

The hijacking of the TWA airliner, which Berri took under his wing on June 15, at first appeared to be a risky undertaking. He agreed to negotiate on behalf of the hijackers on the condition that they promised not to harm the American passengers and provided they narrowed their demands to the release of those Lebanese prisoners moved by Israel to its Atlit Prison. His actions in the crisis transformed an act of terrorism into a political feat with regional if not international impact.

Sources close to him say that Berri is now in a good position to improve relations between Lebanese Shiites and the United States. Other observers see in his skills as a trouble-shooter and negotiator the makings of a political operative capable of acting on a larger stage.

Born to a wealthy family of Lebanese merchants in Freetown, Sierra Leone, Berri showed little inclination for business and trade. He studied law at the Lebanese University, where he was elected head of the student association for four years. After graduation, he went to the Sorbonne in Paris.

Berri dabbled with Arab ideologies as a student and at one point joined the pro-Syrian wing of the Baath party. He is now said to be Syria's number one ally in Lebanon, despite periods of tension and coolness between him and Damascus.

Berri was drawn into Shiite politics in the late 1960s and rose to the front ranks of the supreme Shiite council under his mentor and idol, the missing Imam Musa Sadr.

In 1980, he took over Amal -- the word means hope in Arabic. It has become the Shiite community's vehicle for political assertion, helping to raise it from the lowest stratum of Lebanese society.

Although usually personable and conciliatory, Berri can become forceful and argumentative when challenged. Gentle in manner and an unlikely leader of an undisciplined and often violent group of scruffy gunmen, Berri has built the 7,000-man Amal militia into the dominant Moslem paramilitary force in Lebanon.

Despite the much criticized conduct of his militia in Lebanon's internal quarrels, Berri has established national credibility by consistently emphasizing purely Lebanese issues. He has put distance between himself and Iran, which since its revolution has been a source of inspiration for Lebanon's 1 million Shiites. This policy has alienated some of the more hawkish Amal leaders.

A key move on Berri's path to influence was his call to Shiite members of the Lebanese Army to disobey orders to shell Shiite slums south of Beirut in February 1984. With help from Syria, Lebanese Druze ally Walid Jumblatt and others, Berri's Amal defeated the Christian-commanded Army in Moslem-controlled Beirut. As a consequence, the Army has split, perhaps irreparably, along religious lines.

Initially ignored by Christian Maronite President Amin Gemayel as a political force, Berri was instrumental in bringing about the abrogation of the May 1983 Israel-Lebanon withdrawal accord. In the spring of 1984, Berri was finally admitted into the government of national unity. He became minister of justice, power and hydrelectric resources and of reconstruction, and minister for southern Lebanon.

Yet, despite his many accomplishments over a brief career, Berri has not succeeded in persuading the government to make political changes that would give the Shiite community a larger share in decision-making. The highest political post held by a Shiite has been house speaker. Every president has been a Christian Maronite; every prime minister, a Sunni Moslem.