South African star Theron - who squares off against Stewart's Snow White as evil Queen Ravenna - wore a revealing sheer dress by Dior Couture while the Twilight actress, 22, chose a lacy number by Marchesa, which she teamed with black trainers.
"I love what I'm wearing but I can't wait to take this thing off," said Stewart.
The actress added: "I swear I'm wearing these shoes because I hurt myself. I know that I would look better in the heels."
Oscar winner Theron, 36, was joined by new father Chris Hemsworth, Sam Claflin, Ray Winstone, Ian McShane, Nick Frost, Eddie Marsan and Toby Jones - who all play dwarves - at London's Leicester Square, which was transformed into an enchanted forest.
Australian actor Hemsworth, who welcomed daughter India with wife Elsa Pataky last week, revealed fatherhood is "fun and great".
Theron - who revealed in March she had adopted son Jackson - joked she was not in a position to offer her co-star any parenting tips.
"Oh no, I'm a new parent myself so we're just finding our ways right now," she said.
Rupert Sanders' directorial debut is a modern take on the classic fairytale and sees the feisty princess train alongside the huntsman in the art of war to take on the evil queen.
The film opens in cinemas on Wednesday May 30. Check out the gothic beauties and their co-stars in an enchanted Leicester Square below...