One can hardly gauge the size of the massive mecca of celebritude that writhed and slathered over itself at 50th Street and Sixth Avenue Sunday night, except that it was the night that Radio City was built for: the biggest and most elephantine vaudeville ever created. A night which People magazine would have to create a Sears catalogue-sized issue in order to encompass.

There was nothing to do about it, nothing to say -- it was a mass hallucination. It was also the hallucination of one man -- Alexander H. Cohen, the producer of a hundred shows -- and not that many hits -- who stood in the back of the hall at 6:50 p.m., waiting for the Night of 100 Stars to begin.

With his wife, Hildy Parks, he had conceived and arranged the evening to benefit the Actors' Fund of America centennial, pulled together the first 100 stars -- and as the word of the landslide momentum in the show-business community spread, another hundred and more to arrange to build a new structure for the Actors' Fund home in New Jersey, a retirement home for people in the entertainment industry. "How's it going, Alex?" an old associate grabbed his forearm.

"We'll see," said Cohen. "Get me," he motioned to one of his aides, "my chair."

A young man brought up a plexiglass Z-structured seat, and Cohen sat down and surveyed the territory. Two people walked up behind him and he hopped up to throw his arms around hotel magnates Harry and Leonore Helmsley. They had given a week's free hotel rooms to the cause for all the participants in the Cohen show.

"Here," said Cohen, "are my two beautiful contributors, Harry and Lee. Harry and Lee." He took their hands and led them to their seats. On his way up the aisle, Cohen picked up a lady's scarf from the floor and handed it to her, grabbed two old associates, shook their hands with an impassive tension and, bending his short hulk, bowed to a glamorous woman who kissed him 10 or 12 times in a 30-second period. He climbed back up on his Z-structured plexiglass chair.

"Alex," one of his older aides said, "I just want to take this opportunity to tell you that in all my years of show business I've never seen anything vaguely like this. This is monumental Alex, this is magnificent."

"Good," said Cohen.

Outside on Sixth Avenue, half of the entire street, for a thousand feet up to 52nd Street, was covered with a vast red outdoor carpet that itself was covered with stars ranging from 4 to 6 feet high -- the biggest and tallest names in show business. It was all Cohen's work.

He spotted a young man in the hall, one of the 10 or 12 people in the 6,000-seat auditorium not seated. Cohen wanted everyone in the whole place seated. He had the mayor of New York and $5 million-per-picture stars stuffed away backstage, and he wanted this young man seated.

"Please sit down," he said, "nobody can stand."

The man looked at Cohen as though he had just seen a security guard. "But my seat is behind the camera," he said.

"Please," said Cohen, "be seated."

The most powerful usher in New York led the young man firmly down the aisle and placed him in the seat, then walked slowly back up to his plexiglass highchair and looked over the hall once more.

"Alex! Alex!" said a business friend, grabbing Cohen's impassive hand and yanking it up and down. "Alex, it's magnificent! How's it going?"

Cohen detached himself. He breathed hard and said in a very resolute voice, "We'll see." He reached into his pocket and pulled out a pill, a stomach tablet, and sucked on it. The orchestra struck up. The audience nudged each other and waited to experience star heaven.

Three and a half hours later, the crowd was sated and reeling. Massive numbers had been put on stage, the kind of numbers that could cure a Depression. The first hour and a half of the show had been interrupted and slowed down over and over by long setups and camera adjustments. And the audience had had thrust upon them George Burns, Orson Welles, Tony Bennett, the Doobie Brothers, Robin Williams, Gene Kelly, Christopher Cross, Dudley Moore, Elizabeth Taylor, Lillian Gish, James Stewart, Myrna Loy, Gregory Peck, Ginger Rogers, Paul Newman, Bette Davis and James Cagney. Bodies swayed, people slumped, ladies in long dresses dragged their trains up the aisle. Orchestra seats had cost $1,000 a ticket and the audience couldn't leave. "My God," said a woman, raising her glasses to her forehead, "I'd pay $2,000 to get out of this place."

After Placido Domingo had sung with Miss Piggy, after Ricky Schroeder and Maureen Stapleton had helped Doug Henning saw Florence Henderson and Priscilla Lopez in half and rearrange them, after Liza Minnelli had sung "New York, New York" with a half dozen of the New York Yankees, the audience began to sway as though they had been hit with an odd tropical disease. A couple of men bolted up the aisle sweating. Each time another superstar appeared on stage with a huge birthday cake, lighting one candle a year for each year of the Actors' Fund's existence, the audience began to groan. Starglut had set in.

At theaters around the world, when Liza Minnelli leaves the stage any time before two hours are up, audiences roar their disapproval; when she showed up for a second number in the third hour of the show last night, a troop of disgruntled executives sank deeper into their plush seats. "Oy," growled one, "more Minnelli."

Jack Lemmon showed up, and so did Ed Asner and Rick Springfield. Peter Ustinov lit 10 more candles on the cake, intoning up to the half-century mark. A young man held his head in his hands. "Gee-zus," he said, "fifty more to go."

It was the fourth hour and onstage stood yet another 34 of the biggest stars in entertainment, these from television, singing helplessly about how happy they were that we allowed them to come into our homes: Hal Linden and Loni Anderson and Larry Hagman and Nell Carter. Grant Tinker of NBC sat in the first row of the house giving okay signs and little waves to his network stars.

Lena Horne sang on stage, and dozens of screen couples were reunited: Joel McCrea and Frances Dee, June Allyson and Van Johnson, Brooke Shields and Chris Atkins, Jane Russell and Jack Beutel, Jane Powell and Howard Keel, more and more and more, and finally, Warren Beatty and Diane Keaton, "as you're seeing them now on the screen," came on like the bride and groom on the wedding cake nuzzling and hugging and sending a new wave of life through the masses. "Oy," said one of the businessmen.

David Letterman came and worked with an extraordinary confidence, as the room got hotter. "The Red Cross is coming with blankets," Letterman said. Diahann Carroll sang, and Steve Allen did his old letters-to-the-editor bit and got the biggest laugh in hours: "If a bomb fell on this room tonight," he said, "it would be a big break for Pia Zadora."

"Ladies and gentlemen," Cohen said on the great sound system, booming like Frank Morgan in "The Wizard of Oz," "there will be exactly 23 minutes more." A sigh suffused. It was 11:40 p.m. The last cake was rolled out and Colleen Dewhurst lit the candles.

At 12:05 the curtain lifted, and there was not yet a finale. A man in top hat and tails turned around and sang "Lullaby of Broadway." It was Edward Irving Koch, the mayor of New York. He was joined in a chorus line by: Joel Grey, Carol Channing, Robert Preston, Lucie Arnaz, Robert Klein, Celeste Holm, Alfred Drake, Alexis Smith, Pearl Bailey, Beatrice Arthur, Liv Ullman, Lauren Bacall, Dorothy Loudon, Richard Kiley, Mickey Rooney, Ann Miller, Mary Martin and Ethel Merman, whose arm locked the mayor's as they strutted across what was left of the stage singing "Lullaby of Broadway."

At 12:25, 40 Rockettes rose up on the huge back elevator on the Music Hall stage and began moving in a kick line choreographed by Michael Bennett, singing "One (Singular Sensation)." Cohen stood in the wings. The 40 Rockettes were met by 40 men. "I can't stand any more of it," an usher said, "I can't." The Rockettes were met one by one by Peter Allen, and by Burt Lancaster and by John Vliet Lindsay, and by James Mason, and by Roger Moore, and by Milton Berle and by James Coco and by Al Pacino and by Robert De Niro and by Chris Reeve and by Jose Ferrer and George Segal and William Shatner and Cliff Robertson, by Anthony Quinn, and Andy Gibb. The chorus line spread from Rocky Graziano, flailing giddily, to Lee Strasberg, looking miserable, all kicking, all singing, all shiny as Krugerrands and devalued as deutschmarks.

Alexander H. Cohen stood in the wings.

The curtain opened and the other 180 stars stood on a huge raised bleacher, applauding the audience. "Thank you!" they said, and applauded and applauded. Ethel Merman belted, and Jimmy Stewart swayed in the wind, and Ted Knight waved goonily from the top row. The curtain came down. "Thank you," said the voice of Alexander H. Cohen, from heaven.

Stars began to slip out front, seeping out from behind the curtain.

On the street, a mob waited. New York is huge and hard and doesn't fall prey to Day of the Locust frenzy often. But outside the Music Hall and all the way up to the Hilton on 52nd Street, wild crowds packed themselves six and eight deep behind police barricades that ringed the red carpet. A bullfight roar rose up as Brooke Shields came out, high and smooth as a silver skyscraper, draped in bronze sequins and a cape, her leonine hair frizzed to Sheena-the-Jungle-Girl tangles. Roars came up in bunches from the crowd and photographers yelled "Brooke, here!" and "Brooke, here!" as her mother Teri Shields directed here and there and here at the cameras toward which she had to smile. The crowd grew greater and deeper and madder up close to the Hilton and finally, as Brooke fought her way up near the windows that led to the revolving door, the mob finally reached Nathanael West proportions.

The yelling and the pressure got higher and louder and movie stars were turned around rolling in it like tumblers in a wall safe lock. Brooke was guarded on all sides by her mother, by Melissa Gilbert, by security men and all the while she kept smiling -- pushed in from all sides though she was, crammed and mauled. "Brooke, howdja get so big?" Suddenly she was hurled into the hotel and onto the escalator and she stood floating up above the crowd that kept yelling "Brooke!" for her and she kept a cool, kind smile as she was carried higher and higher and then in a moment she dissolved. It was as though her face had been washed away by a rain and she was in tears, uncontrollable tears. She cried and cried and she stood on the landing above the escalator at the Hilton and cried more, her superstructured body dipping in small spasms. "Why do they have to do this to us," she cried, "why do they do this?"

Teri Shields patted her on the back. "They're just people trying to be nice," she said.

Brooke Shields cried while raising Kleenex to her face, blotting it, and then suddenly, as if the rain had passed, she was standing straight, high above the crowd around her, smiling pleasantly, very pleasantly once more at anyone who wanted to see her.