Milton Nascimento: cultural ambassador of popular music.

A black man in a floppy denim cap and an airy white suit walked onto the stage at Carnegie Hall singing an eerie a cappella. Nearly two hours later, Milton Nascimento was straining to be heard over the explosions of applause, and the audience in the stately New York concert hall had poured out into the aisles to sing and dance.

The Brazilian singer and composer, who played to two sellout crowds in New York earlier this summer, hadn't uttered a word in English. He had hardly even spoken between songs. Rather, he did simply what he has done for most of his nearly 25 years as a performer. With his molten tenor and soaring falsetto, backed by a disciplined four-piece band, Nascimento created a nearly mystical communion with his audience that demolished the barriers between his continent and this one.

In a sense, Nascimento, now 41, is as much a cultural ambassador as he is a musician. Though his compositions draw from the rich and varied i of Brazil, he rejects the narrow label of ''Brazilian musician.''

''I only see the possibility of global music, people everywhere struggling to put forward and share their own ideas,'' he told this writer.

In fact, it is Nascimento the troubador-diplomat who recently completed what was perhaps his most ambitious concert and recording tour - to Canada, Denmark, France, Germany, Sweden, Austria, Spain, and Italy. On his drawing board are plans for a number of cross-cultural ventures. He will record two albums, one with jazz guitarist Pat Metheny and another with popular jazz artist Wayne Shorter, and he will collaborate on a ballet with choreographer Alvin Ailey, to debut this year.

His tour comes at a time when, he senses, there is a readiness to hear Brazilian music in all its complexity. ''I think people know now Brazil is not just the land of snakes in the street, bananas, and jungle,'' he chuckled as he sat among friends and colleagues in his New York hotel room.

Nascimento's music is a celebration of the diversity of his native land. On any one album a listener might find complex Afro-Brazilian percussion combined with the breathy churchlike harmonies of his home state, Minas Gerais. Or a song in the fashion of the Iberian balladeers might be melded into samba rhythms and followed by a near Gregorian chant. Led by the finely textured keyboard work of longtime colleague Wagner Tiso, Nascimento's group can sound at one moment like a bossa nova big band and at another like the most solemn of choirs.

If this versatility won him acclaim abroad, it has propelled him toward introspection and retreat at home. Two years ago, against the advice of friends and artists, Nascimento padlocked his Rio de Janeiro apartment and moved away from the hot lights and back to Minas Gerais, the mountain-bound state in central Brazil. ''In Rio,'' he said, ''I was living for the profession and not for my own life. My life was being dispersed.''

Today, Nascimento's office is a simple two-story colonial home in the capital of Minas, Belo Horizonte. There are no wall-to-wall mirrors, no deep-pile carpeting, no throbbing stereo system, nor any of the paraphernalia of corporate superstardom. He calls it Quilombo, after the egalitarian communities blacks established after they escaped their Portuguese slave masters. It is also a haven for Nascimento's Projecto Musica de Minas, a multilayered musical venture designed to revive interest and creativity in Brazilian music. The project includes an experimental music school, for students aged five and up, taught by performing musicians. And just recently, Quilombo christened a weekly radio program featuring traditional and contemporary Brazilian music, airing on 17 Minas Gerais radio stations - no small accomplishment in a market saturated by American and European rock music.

Through most of his professional life, Nascimento has in fact sought to counterbalance corporate domination of music. It hasn't always been easy going. It took 10 years for producers to accept the work of his own editing company, Editora Tres Pontas, founded in 1967. He produced what is considered to be the first independent album in Brazil in 1962. (Now there are over 300 independently produced lps on the Brazilian market and some 80 percent of musicians retain editing rights over their music.)

Many of Nascimento's songs are politically inspired, something the military-backed Brazilian government did not always take to. Once a virtual persona non grata, he was obliged to record one album, ''Milagre dos Peixes,'' virtually without lyrics. But Nascimento has remained committed to social causes , and three of his latest songs became virtual hymns of the recent surging popular movement calling for direct elections for president.

Two years ago, Nascimento also bucked industry protocol when he signed an extraordinary contract with Ariola records (owned by Polygram), demanding that in turn for recording another Nascimento album the company would agree to produce albums by eight unknown, unrecorded Brazilian musicians. Four of these records - by Celso Adolfo, Tadeu Franco, Tulio Morno, and Noveli - have already been produced.

For all his accomplishments, Nascimento wants still more. He worships his own artistic idols as passionately as many of his fans worship him. He longs to meet Francois Truffaut, whose film ''Jules and Jim'' moved the then 19-year-old to cease crooning imitations of other people's music and strike out on his own. Nascimento also wants to make films of his own (he's participated in a number, including Werner Herzog's ''Fitzcarraldo''). But, he confesses, ''the dream of my life'' is to record with legendary jazz trumpeter Miles Davis. ''I feel Miles playing as if I were singing.'' Chances are, if that venture comes off, Nascimento will once again have audiences on any number of continents applauding , singing, and dancing in the aisles.