Thousands of fans packed Leicester Square in London last night to catch a glimpse of the cast of Spider-Man 2, tipped to become the summer's blockbuster film.

The stars of the film, Tobey Maguire and Kirsten Dunst, were briefly upstaged, however, when the singer Samantha Mumba appeared in what is thought to be the world's most expensive dress, a skimpy, diamond-studded garment worth £5m.

She insisted the dress, which was designed by Scott Henshall and used 3,476 diamonds, was "really comfortable, actually".

However, she was quick to swap the 423.68-carat creation for a black crocheted minidress, also by Henshall.

The film, which follows Peter Parker's struggle to reconcile the pressures of young geekhood with being a web-slinging arachnoid crimefighter, broke all records when it opened in North America, taking more than $200m (£109m) in its first eight days. It opens across the UK on Friday. Maguire, who plays the troubled superhero, said he was overwhelmed by the size of the crowd.

"It's amazing that so many people have turned out for this premiere," he said. "The film is pretty great and I'm really proud of it."

Dunst, who plays the object of his affections, Mary Jane Watson, turned up in a white one-shouldered dress by Chanel.

"I don't mind about making the front pages. I just want to wear something beautiful," she said.

Other celebrities who were drawn to the red carpet like bluebottles to flypaper included Lenny Kravitz, Claudia Schiffer, Jenson Button, Rachel Stevens, Justin Hawkins of The Darkness, and the Baywatch star David Hasselhoff. Less welcome was the presence of Aaron Barschak, the self-styled "comedy terrorist" who gatecrashed Prince William's birthday party last year.