"I think it's a great idea," said Stone, donning a black Cavalli dress and black boots.
The Cavalli H&M collection will offer 25 outfits for women and 24 for men, plus accessories and lingerie - all with low-end price tags.
"H&M has given me the opportunity to make a present to all those young people who can't afford my signature clothes," Cavalli said at a news conference earlier Thursday.
Models dressed in the H&M Cavalli creations, fashioned for the most part in the designer's trademark animal prints, mingled with the 1,500 guests who dined on Italian specialties and danced into the wee hours on the red-carpeted runway, in a venue decorated to recall a 1960's Roman nightclub.
"Cavalli's fashion is not only about red carpets, but about a positive and optimistic attitude," said Margareta van den Bosch, head of design at H&M.
Van Bosch said the collection will be available from Nov. 8 in 200 of the Swedish company's 1,400 stores. Once sold out it will not be reproduced.
The collection's most expensive item is a EUR299 (US$428) gold lame evening gown. One of the cheapest is a pair of men's jeans for EUR59.90 (US$86).
H&M has teamed up in the past with designers Karl Lagerfeld, Stella McCartney, and Viktor&Rolf, as well as pop star Madonna.
"It's a magic moment I will never forget," Cavalli said as he arrived at the party, puffing on a cigar as he walked down the runway accompanied by a bevy of cheering models decked out in H&M by Cavalli.