One hundred voices sing a salute to 100 years of the Met

How does a world-famous opera company celebrate its 100th birthday? By inviting most of the biggest names in opera - past and present - to share in a day of singing and festivity. And then it allows TV cameras in so a video document can be made for posterity and so that the world can experience the event live.

What was most wonderful about the gala?

Hearing all the great singers in one day.

And what was most disturbing?

The realization that most of the great singing came from artists past their prime. These artists have sustained the tradition passed down to them. The younger singers heard during the eight-hour marathon Oct. 22 are not, with a few obvious exceptions, the sort that will be continuing the tradition - a fact which surely the Met did not intend to highlight on its 100th anniversary.

The day was planned in two four-hour segments, 2-6 p.m., then 8-midnight. Nearly 100 singers were invited to participate. Suffice it to say that most of the superstars and legendary artists singing today were included. A few, such as Leonie Rysanek and Christa Ludwig, were unable to attend - and others, such as Jon Vickers, chose not to sing for personal reasons.

The obvious ''household'' names like Luciano Pavarotti, Placido Domingo, Dame Joan Sutherland, Leontyne Price, and Birgit Nilsson were backed up by a stunning array of superb artists who have defined and sustained the Met's lofty operatic standards. And the distinguished American backbone of the Met roster - those that were always on hand to do a role justice and fill in at a pinch, such as Jerome Hines, Lucine Amara, and Robert Merrill - were featured as well.

The chance to hear so many singers in eight hours was enough to emblazon this day in musical history. One could thrill to Eva Marton, who began the day's singing with a hair-raising ''In questa reggia'' from Puccini's ''Turandot.'' Had the rest of the singing been equally extraordinary, the event would have been something posterity would have looked back on as a resplendent new golden age.

Not that there weren't moments throughout the two segments of unusual magnificence: Nicolai Gedda's haunting, masterly ''Una furtiva lagrima'' (from Donizetti's ''L'Elisir d'amore''), Anna Tomowa-Sintow's melting, limpid ''Ernani , involami'' (from Verdi's ''Ernani''), the love duet from Verdi's ''Otello,'' sung by Mr. Domingo and Mirella Freni, Miss Sutherland's ''Bel raggio lusinghier'' (from Rossini's ''Semiramide''), and Regine Crespin's delectably insinuating ''Habanera'' (from Bizet's ''Carmen''). Montserrat Caballe and Jose Carreras gave a rousing performance of the final duet from Giordano's ''Andrea Chenier. Then there was Jessye Norman's handsome Sieglinde, Neil Shicoff's rousing Hoffmann, and Catherine Malfitano and Alfredo Kraus's ardent ''Romeo and Juliette'' duet.

Interspersed throughout these things of note were scenes, arias, and ensembles sung by the supporting singers that make up the backbone of the Met roster. But these hardly did justice to the Met at this historic time or on any other occasion, and they took up a huge part of the evening session.

Several of the Met's most handsome sets were used to frame the segments of each session: Franco Zeffirelli's second act ''La Boheme,'' Gunther Schneider-Siemssen's ballroom for Strauss' ''Arabella,'' Robert O'Hearn's third-act ''Samson et Dalila'' (brought on just for the ballet), Marc Chagall's ''Magic Flute.''

The orchestra was in generally good form, particularly under music director James Levine, who presided over much of the gala. The magnificent chorus was given its special moment - the ''Hymn to the Sun'' from Mascagni's ''Iris.'' The ballet was unfortunately showcased in Zachary Solov's thigh-slapping, arm-waving ''Bacchanale.''

The gala was notable for the return of several singers who, for various reasons, had not been expected to be heard at the Met again. These included Mme. Nilsson, James McCracken, Martina Arroyo, and Miss Crespin. A quantity of famous singers who are no longer performing were invited as honored guests. In the last third of the evening session, those who attended were seated on stage to hear and be seen. These included some of the greatest singers of their day - Zinka Milanov, Jarmila Novotna, Dorothy Kirsten, Erna Berger, Rose Bampton, Bidu Sayao , Ferruccio Tagliavini, Eleanor Steber.

Their presence was one of the few reminders all night long that this was supposed to be a celebration of 100 years of operatic tradition.

When Mme. Nilsson sang a charming Swedish folk song favored by Christine Nilsson (who opened the original Met in 1883) this link to the past made a profoundly touching moment - in fact the most moving moment of the night. Suddenly present and past were fused in a magical suspension by one of the legends of vocal history.

Throughout, the feeling was of a thoroughly modern company honoring its current profile. When the big stars were involved, this was fine. When members of the young crop of house singers were featured, the gala - which grossed $1.45 million - was seriously wanting in stature.

Nevertheless, the last sounds that echoed in the house at 1 a.m. were the dying strains of the most lavish ''Happy Birthday'' chorus ever heard - all those singers blending in energetic harmony with the Met chorus.

That was a thrill!