CELLULAR PHONES WERE BUZZING AT restaurants around town; fax machines hummed and harried publicists scrambled for spin control. Last Tuesday, just three days before Hollywood's youthful lovebirds of the year, Pretty Woman star Julia Roberts, 23, and Young Guns' Kiefer Sutherland, 24, were to become man and wife on a 20th Century Fox soundstage, they unceremoniously called the whole thing off.

"It has been mutually agreed upon that the wedding has been postponed," announced the stars' publicists jointly. The cancellation left some 500 guests wondering. While rumors flew about what caused the romance to sputter (if it did), a source close to Julia says, "It was Julia who called the wedding off. She has wanted to do it for some time but didn't know if she could find the courage." Uncertainty about marrying, intensified by embarrassment over mounting publicity, pushed her to the decision, the friend says.

The two fell for each other on the set of Flatliners in 1990, when Sutherland was still married to his first wife, actress Camelia Kath, 37, mother of his daughter, Sarah, 3. (He and Kath were divorced in early 1990.) Almost from the start of their romance, Roberts and Sutherland seemed to be Hollywood's reigning couple. Sutherland eventually moved into Roberts's million dollar house in Los Angeles, and in March she began wearing a costly diamond ring. But in May the tabloids linked Sutherland to a 24-year-old stripper, Amanda Rice, an allegation he vigorously denied.

Last week, while promoting her new movie, Dying Young, due out June 21, Roberts was to have had her last fitting at designer Richard Tyler's Los Angeles shop, Tyler Trafficante. Now the custom-made wedding gown waits unclaimed, along with four seafoam-green silk bridesmaids' dresses. And Dominick's restaurant in West Hollywood had a last-minute cancellation on Kiefer's June 11 stag party. As for the wedding gifts, Dying Young director Joel Schumacher, a close Julia pal, says his present will "stay in the closet till I hear new things." Or maybe till Beverly Hills freezes over.