When Rosa Ponselle, the great Metropolitan opera star of the '20s and '30s, woke up on Saturday morning, she expected about 10 of her close friends would be coming in for an 80th-birthday spaghetti supper that night.

Her companion, Elayne Duke, said. "I kept insisting to her that she should not ask anyone else in - just to keep it nice and small."

"Nice and small" turned into a festive crowd of about 110 people for a gala surprise celebration that filled every corner of Villa Pace, the Italian mansion in the Baltimore suburb of Green Spring Valley that has been Ponselle's home for the 40 years since she married Carle Jackson, when he was the son of the Baltimore mayor. The marriage ended after a few years, but Ponselle has never moved from her hilltop spot.

Thousands of Ponselle lovers wondered for years why their idol never returned to sing at the Met. Her departure six weeks after her 40th birthday, with her voice still in magnificent form, left everyone wanting more.

The stories were that she was hurt by unfavorable reviews of her Carmen, and that when she asked for a new production of an opera she had never sung, the Met management said it could not afford it during those post-Depression days.

Though she sang regularly in concert and on the radio after leaving the Met, her final notes there were sung at a Sunday evening concert on March 14, 1937.

Saturday night's surprise stayed a secret until around 6:30 when Francis Robinson, vice president of the Metropolitian Opera, and a longtime friend, admirer, and annotator of the Ponselle story, walked in.

"My God," Ponselle exclaimed when she saw him, "What's going on?" Following close on Robinson's footsteps came such former Met guests as Licia Albanese, one of the brightest Violettas ever to follow Ponselle in Verdi's "La Traviata."

Rose Bampton was there, remembering her own 23d birthday on Nov. 28, 1932, when she made her debut at the Met. singing Laura, opposite Ponselle in "La Gioconda." The American mezzo that night was just two years older then Ponselle had been when she made her famous debut opposite Caruso at the age of 21, and Bampton's reviews were as glowing as those that had greeted Ponselle.

Nanette Guilford came in with memories of the Sunday night gala concerts on which she often appeared with Ponselle. In her Met career, a decade when Ponselle was at her peak, Guilford had sung Ines in Meyerbeer's "L'Africaine," when Ponselle was the Selika.

Stella Roman, one of the best dramatic sopranos to take over such famous Ponselle roles as Aida, Santuzza, and Lenora in "Trovatore," was there. And conductor Wilfred Pelletier, Bampton's husband, who often conducted for every one of the singers at the party, showed up too.

Duke had arranged the whole evening without letting a hint of it get to Ponselle. There were Baltimore television cameras and interviews for which Ponselle talked about her early days in Meriden, Conn.

"My piano teacher was rthe church organist," she said, when someone wanted to know where she learned to play well enough to accompany herself and her mezzo sister, Carmela, in their appearances on the Keith vaudeville circuit.

Around 9 o'clock, some glorious vocalizing began to be heard in brief spurts over the nonstop conversation. And sure enough. From the powder room just off the entrance hall came a series of rising notes, incredibly rich in texture, firm, secure, and powerful. It is not possible for a woman 80 years to sound that way, but Ponselle has never been limited to what was possible. Her speaking voice today has the same substance, the same vintage timbre.

Out she came, and everyone crowded around her in the foyer. Suddenly another operatic voice began, "Happy birthday to you, Happy birthday to you."

Everyone joined in. At the end, some of the most famous voices in operatic history soared up to a high B flat or whatever it was.

"Would you all mind doing that again," came the voice of a TV crew member. "We had the cameras on but the mike did not get it quite right."

So now, everyone knowing that his best notes were being recorded for relative posterity, out came the "Happy Birthday" again. And again the high note, this time very definitely B Flat.

"Once more, if you don't mind," said the voice of television. And obediently, as if they couldn't have loved it more, the "Happy Birthday" rolled. At the top notes, stronger than ever, Ponselle exclaimed "Aha! Another star is born!" Just which star may never be known.

Then came the great moment as everyone turned absolutely quiet. Ponselle began to sing, with no accompaniment: "I love you truly, truly dear. All of my friends here. . ." and then the voice stopped. Was the emotion of the moment too much?

Not a bit of it. "I have been dissipating," said the greater singer, who has always maintained a strict discipline over her eating and drinking habits. "I can't sing when I have been dissipating."

But it may have been the excitement of the evening, too. Frequently as her friends piled into the house, there had also been phone calls - Elisabeth Schwarzkopf called from NiceM France, in the middle of the day - and telegrams and taped messages from those who could not be there, had come in like a snowstorm.

While Ponselle was enjoying the lavish supper which her friends had arranged, a long-distance phone call came from Meriden, Conn., froma grade school classmate, Juliette.

"No dear" Rosa said through tears, while she talked. "I am only crying from happiness. It is all like a dream come true." It was the right kind of dream for someone whose singing made beauty for over 50 years.