His sudden propulsion into the vortex of the hijacking drama has confirmed what Beirut watchers have known all along - that Nabih Berri, a soft-spoken, mild-mannered, 46-year-old lawyer is perhaps the most powerful man in Lebanon.

His leadership of the mainstream of the Shiite movement gives him political power that outstrips that of the Christian President, Amin Gemayel. As head of the Amal militia, he has battlefield strength to contend with the Palestinians or the Druse fighters of Walid Jumblat.

Mr. Berri, who retains a green card entitling him to work in the United States and whose first wife and six children live in Dearborn, Mich., long plotted a steady, moderate course. It was not until Israel invaded Lebanon in the summer of 1982 that he came into his own as a political force.

Mr. Berri was gradually radicalized by Lebanese politics. His crowning success came in February last year when his militia overpowered Lebanese soldiers in West Beirut, destroying the credibility of the American-backed Lebanese Government and proving that he could perform as a tough-minded military man.