"This is the most impressive gathering of stars any of us will ever see," Steve Allen told 5,400 in New York's Radio City Music Hall last week. "If a bomb fell on this room tonight, it would be a big break for Pia Zadora." The audience had spent up to $1,000 for admission to the show and attendant celebrations, and they were getting all that was promised—and more—at the Night of 100 Stars. Actually, twice that number of luminaries showed up, and their scheduled three-hour TV extravaganza took six hours to tape. The occasion was a benefit for the 100-year-old Actors' Fund of America, which assists ailing and impoverished performers. The goal: to raise enough to build a nursing home in northern New Jersey.

For the show, Princess Grace and Elizabeth Taylor talked about the history of the fund, Brooke Shields and Morgan Fairchild modeled, Christopher Cross sang two of his hits, and Paul Newman and Al Pacino merely walked across the stage. Of course, everything did not happen on cue. George Burns missed his, James Earl Jones' TelePrompTer went blank and Orson Welles' microphone died. But on March 8, ABC's version will be a smooth and seamless evening of entertainment. It was 12:30 a.m. when the last curtain call was taken—"I hope you all live as long as this evening has turned out to be," Alan King joked—and 5 a.m. when the subsequent dinner dance at the Hilton broke up. "It may seem like a night in Hollywood, but it feels like a day in the Ukraine," quipped one hardy fan waiting outside the hotel. But the true star of the show, as PEOPLE writers Gioia Diliberto and Fred Bernstein discovered, went largely unrecognized. He was Alexander Cohen, the gaudy ringmaster of the event.

Producer Alexander Cohen's unusually high number of Broadway flops has earned him the title "hitless wonder" in certain circles. But when it comes to producing hype, Cohen, 61, has no peers. He's at his best masterminding television specials like the annual Tony Awards, the 1978 Emmys, A World of Love for UNICEF and, of course, Night of 100 Stars. A flashy figure in a custom-tailored suit and stretch limo, Cohen is one of the last of the big schemers. "I've been flat broke five or six times," he likes to boast. "I couldn't afford a car and chauffeur, but I still had them."

To underwrite the Actors' Fund Home, Cohen needed the biggest names in the business, and to get them, he cajoled New York's carriage-trade merchants into donating their goods and services. The swank Helmsley Palace Hotel provided 100 rooms and suites—including a triplex with six marble bathrooms, two wraparound terraces and a formal dining room for Princess Grace. Liz Taylor's daughter Maria found Mom's suite perfect for her pregala wedding to talent agent Steve Carson. Three airlines gave the far-flung celebs first-class tickets to New York, restaurants offered complimentary meals, and Harry Winston loaned $50 million worth of diamonds and rubies to a dozen actresses, including Ginger Rogers, Helen Hayes, Lillian Gish and Cher. To transport the stars, who performed gratis, seven limousine fleets donated cars and drivers. Exulted Cohen: "If I were paying for all this, it would cost me millions."

It took Cohen and his staff of 28 seven months to plan the affair. He sent out 260 invitations, hoping to attract 100 stars; when 218 accepted, the producer declined to change the name of the show or be daunted by the logistics. He supplied free hairdressers, masseurs, makeup artists and a 24-hour messenger service for each celeb. "If you have forgotten anything from a toothbrush to medication, we can handle it. And what we don't have, we can find," read a note left in each room. Cohen also arranged an "extra-length" bed for 6'3½" Jimmy Stewart, sent one of his minions to pick up Dudley Moore's theater tickets, and kept a Warner jet on call for any celeb who suddenly got ill or bored. In each star's room, "placed at appropriate, discreet places," were Rémy Martin cognac, bouquets, fruit, perfume, Godiva chocolates and record albums (Evita, 42nd Street and The Pirates of Penzance). But even Cohen knew when to quit. His wife of 26 years and co-producer, Hildy Parks, suggested adding more loot but Cohen objected. "Now, darling," he snapped, "don't you think this is getting vulgar?"

In his windowless office on the top floor of the Shubert Theatre, the impresario put in 14-hour days, seven days a week, the month before his big night. Still, Cohen claims he won't make any money on it. While his staffers wrote scripts and orchestrated parties, hotel accommodations and transportation, Cohen played the telephone like a virtuoso. In five minutes one afternoon he took four calls in rat-tat-tat succession: "Hello, we need Robert Culp in New York on Saturday.... If I can't have the whole joint tell Regine to forget it.... Hello, no, Julie Harris is not lighting a candle.... You should know there's a possibility of Streep showing up." (She didn't.) Until the last minute, Cohen struggled to add stars to his firmament. A few days before, an agent called to say Dustin Hoffman couldn't make it because of a meeting in California. Cohen erupted: "All I'm asking is for one Sunday out of his life.... Tell him I'll send a plane for him.... Listen, the only excuse I'll accept is a coronary." Nonetheless, Hoffman was a no-show.

Cohen, the son of a clothes manufacturer who died when Alex was 4, grew up on Park Avenue but even as a child was enchanted by Broadway. "When I was 6 or 7 I had a recurring dream that I was walking down the aisle of a theater to enormous applause," he recalls. "Yet I'd never even been in a theater." Cohen produced his first event in 1933, a carnival at Manhattan's Columbia Grammar and Preparatory School which, he insists, raised $31,000 for New York's Fresh Air Fund. Eight years later he blew a $65,000 inheritance on a barn theater in Long Island. In 1941 he also made it to Broadway with Angel Street, until last year the longest-running thriller in New York history. When subsequent ventures failed, Cohen was forced to take a job as publicist for the Bulova Watch Company. In 1946, hearing that the Motion Picture Academy was insolvent and about to lose its Los Angeles headquarters, he persuaded Bulova to pay off the academy's mortgage in exchange for the license for "Academy Award Watches" with Oscar on the face. Cohen says Bulova made $28 million on the timepieces in 1947. Ever since, Cohen has lived handsomely off show business. While big-money investors avoid his projects now, he manages quite nicely. His secret: "You have to be flamboyant."

He lives accordingly. He owns ritzy homes in New York, London and Cagnes-sur-Mer in the south of France. But the producer is most at home in the wings of a theater. At Radio City he was on hand, sweating and waving wildly as he directed performers on-and offstage (there were only two all-day rehearsals). Even when a curtain was lowered accidentally on Elizabeth Taylor's head and a TV cameraman nearly clobbered a Rockette while taping the kick line, Cohen refused to despair.

The audience gave every star a standing ovation, though possibly this was just an opportunity to stretch tired legs. But when the Rockettes led 36 male celebs including Ed Asner, Robert De Niro, Lee Strasberg and Christopher Reeve in a series of precision kicks, the applause was explosive. At the finale all the performers massed onstage in a burst of color.

Afterward the cast left the theater to walk three and a half blocks to the Hilton on a 6,000-square-yard red carpet. Alexander Cohen, of course, was walking on air. Yet some guests were plainly relieved that Night finally was over. Sighed designer Vera Maxwell, 80, who had fought to stay awake: "The trouble with Alex is, he always does everything too big."