Lynne Bredfeldt had to deal with some aggressive shoppers physically forcing their way through the racks. "A woman was pushing me saying 'That's mine!'" she said. "People were grabbing all the sizes, taking three or four pieces at a time."

Bredfeldt used her own strategy and camped out near the dressing room in search of discarded merchandise. It worked. She left with two sought-after pieces, an animal-print trench and a sheer animal-print tunic.

H&M tried to ease the discomfort of those waiting outside in the cold. At the Fifth Avenue and 51st Street flagship in Manhattan, the first 150 people on line received gift bags with limited edition Cavalli T-shirts, autographed images from the ad campaign and gifts cards from $10 to $300. In Milan, crowds were offered trays of hot croissants, orange juice and water, and those outside the H&M Oxford Circus flagship in London found croissants and orange juice packaged inside Cavalli goodie bags.

By late afternoon, a big red sign outside the Oxford Circus H&M unit stated the collection was "Sold Out," which was not surprising, considering crowds were four deep and the line wound its way around the block. When the doors opened at 9 a.m., a DJ blasted tracks while customers ran in every direction. "It's like descending into a leopard-print ant farm," quipped one consumer as she made her way down the escalator to the basement, where shoppers were snapping up Cavalli's signature animal prints.

The entire shop floor was transformed into an impromptu changing room for men and women trying on tailored suits, evening gowns and shoes. Other customers came prepared for bulk spending, bringing suitcases and plastic hampers.

Among the top sellers in London were leopard-print kaftans, heavily-sequined minidresses, leather jackets, accessories and long gowns. Racks of some items — including the print kaftans — were cleared within seconds. Ten minutes after the opening, the basement of the Oxford Street store was clear, save for a few pieces.

Some London units, including Knightsbridge, began waiting lists for customers seeking specific items. The frenzy was attributed to the limited quantities. Staff members said stores received 100 percent of their stock allocation in deliveries the night before, with no replenishments to follow, in contrast with previous Stella McCartney and Viktor & Rolf collaborations, where top-up deliveries were made throughout the day.