The sixth Potter film, "Half-Blood Prince" sees Harry and his friends at Hogwarts school battling evil Lord Voldemort — and their raging teenage hormones.
Fans cheered the film's stars despite the weather, some holding placards reading "Harry, I'll be Your Chosen One" and "Britain Loves J.K. Rowling."
"I can't believe the dedication of the fans," said Tom Felton, who play's Harry's rival, Draco Malfoy, in the film.
Paul Taylor, 45, a nursing-home worker from Sheffield in northern England, came dressed in a tailor-made wizard's cloak and hat.
"It's quite exciting. You get caught up in all the energy," he said.
"I get quite involved with Harry Potter," he acknowledged. "I travel on steam trains and go to conferences."
Grint, who plays Ron Weasley, attended after recovering from a mild bout of swine flu. He said the illness was not severe and "just like any other flu I've had before."
The cast members wore white ribbons in memory of Rob Knox, a young actor on the film who was stabbed to death in London last year.
"He would have loved to be here," said Jessie Cave, who plays Ron's sometime love interest, Lavender Brown.
The movie opens around the world July 15.
Two more films are in the works covering Rowling's final book in the series, "Harry Potter and the Deathly Hallows."