The Fifth Annual Spoleto Festival is in full swing, and there is a particular air of celebration because its founder, director and moving force, Gian Carlo Menotti, will soon mark his 70th birthday. A grand pre-birthday party was givven Menotti last night at Seabrook, the home of John Kessler, president of the Spoleto board of directors. Sharing the spotlight with the composer was soprano Renata Scotto, who had just done a recital before a huge audience that raised the roof for the popular diva.

Major Menotti works are giving this year's festival much of its greatest excitment. And there is much more. Beginning last Friday and running through June 7, more than 100 artistic events of all kinds will be seen and heard, with world premieres of operas, plays and dance. Many more events are running simultaneously in the Piccolo Spoleto, which includes the Fringe Festival and smaplings of the visual, graphic, musical and literary arts.

Still, Menotti's opera "The Last Savage," which had its world premiere in Paris in 1963 and its United States debut at the Metropolitan Opera the following January, is the centerpiece. It is being presented four times in Gaillard Auditorium in a spectacular production by Beni Montresor, the designer of the Washington Opera's recent triump, "The Love of Three Kings." The sets and costumes for ""Savage" are a dazzling array of colors that provide the perfect background for the witty satire on today's civilization and society that is the core of Menotti's story and music.

The opera did not do well when it was new in this country, but today it is hard to see how this production would be anything but a smash hit. The text, largely in rhyming couplets, takes aim on the pomposities of what is called "culture" by the snobbish elements in the arts, but does it with wit and grand humor. More important, the music is some of the most sophisticated and ingtenious in the entire Menotti repertoire. It's arrangement of display arias of all kinds and their intermingling with ensembles from duets to septets often reminds listeners of "The Magic Flute," a parallel that is heightened by the oriental setting.

Nowhere else has Menotti explored the wide range of choral and orchestral writing that is heard in much of "The Last Savage." A stunning fugue is skillfully worked into the score with such ingenuity that it would never disturb those who might think a fugue an intellectual hazard -- shades of Wagner and Verdi! The last-act septet is of surpassing beauty, reminiscent of the septet in Berlioz's "Trojans."

Christian Badea, at one time Exxon/Arts Endowment conductor of the National Symphony, is a superb conductor of the opera, leading an expert orchestra and chorus. The outstanding artistic "find" of this year's festival is a young Korean soprano, Suzanne Hong, whose name and voice are destined to become widely known soon. There is a rare texture to her lyric voice, and she has an unusual flair for acting. Both in the opera and in a Menott choral concert, she caught everyone's attention.

The entire cast is well balanced. At the end, the plot turns pure Gilbert-and-Sullivan concealed identity, a tradition almost as old as opera itself.

Those who often tried to put Menotti down as old hat or repetitive or in other critical ways would have difficulty convincing anyone in Charleston's audiences these days that Menotti is not one of the most inspired composers around. This feeling, strongly induced by "The Last Savage," is powerfully heightened by the concerts of two recent choral-orchestral works that have extraordinary beauty in test and music.

The first is an autobiographical work called "Landscapes and Reminiscences." It is a cantata of nine episodes covering the 45 years of Menotti's life in this country, from his arrival at the age of 16 to the time he left for his new home in Scotland. It is filled with ingenuities for both chorus and orchestra and for eloquent solo passages. If the scene describing a Texas parade is pure American honky-tonk, the ghostly episode in an abandoned South Crolina mansion makes a paticular effect here in Charleston. There is an exquisite scene recalling Menotti's life in Philadelphia and his meeting thre with Samuel Barber and Barber's aunt, Met star Louise Homer.

The second work is a mass, "O Pulchritudo" ("For Beauty"), that Menotti wrote three years ago for the Bel Canto Chorus of Milwaukee; it is a score strangley neglected by choral groups. Cast in the tradition of masses employing large orchestra, chorus and soloists, it is filled with some of Menotti's loveliest and most striking music. The Agnus Dei is a passage of overwhelming beauty in which the plea for peace is left unresolved.

Chamber music concerts at these festivals have alway been among their strongest attractions and this year is no exception. With Charles Wadsworth as a master of ceremonies, a role he also fulfills in Spoleto, Italy, and in which he is without an equal, one concert reached a moment of passionate beauty in a performance of Tchaikovsky's "Souvenir de Florence" that is unmatched in my experience.It was played by the Emerson String Quartet, which was joined by violist Scott Nickrenz and cellist Laurence Lesser