"My dress is so tight," she said at the Leicester Square unveiling. "Getting up the stairs was a lot harder than any of the stunts we did in the film."
The actress was joined by Kate Beckinsale, gorgeous in a black Donna Karen gown with a daring thigh-high split, and Colin Farell.
All three star in the movie, a remake of the Nineties sci-fi film originally starring Arnold Schwarzenegger as a construction worker in 2084 who finds out that his comfortable life is not all it seems.
Colin said that Underworld star Kate and Jessica were "lovely" to work with and said the three of them "had a good time" filming together.
"They are both tough girls," he added. "Kate has been fighting vampires for centuries and Jessica grew up in the wilderness."