By Tim Masters
Entertainment correspondent, BBC News
The Potter stars were mobbed by fans on the Millennium Bridge
Stars of the Harry Potter films have spoken of the "electric" atmosphere on set, as filming for the final two instalments takes place.
"I've done a few days which went fantastically," said Matthew Lewis, who plays student Neville Longbottom.
Lewis was speaking at the launch of a competition that offers a role in Harry Potter and the Deathly Hallows.
Lewis and co-star Evanna Lynch (Luna Lovegood) released 1,000 black balloons from the London's Millennium bridge.
The bridge features in the explosive opening scene in the sixth film, Harry Potter and the Half-Blood Prince.
Each balloon has a ticket with a code that entitles the holder to enter a prize draw. The winner will appear in the final film as a moving portrait on the walls of Hogwarts school.
Lewis said: "The winner will be able to see all the stuff behind the scenes. People always assume the bigger sets are all computer-generated, but a lot of them are actually there."
The Half-Blood Prince DVD is due in December
Commenting on the rule that entrants must aged 18 or over, Lewis said: "Obviously we started off as primarily a kids' film and it's a shame that it's over 18s, but we've got a massive demographic now.
"The films are accessible to all ages. There's a lot of people now that can really get into the films and are really going to enjoy this prize."
Lewis has been working on and off on the final two parts of Deathly Hallows - which are being directed by David Yates.
"You wouldn't be able to tell it was the last in the series because the atmosphere is electric - everybody is having such a great time," said the 20-year-old star.
Evanna Lynch, who plays Harry's dreamy friend Luna, told the BBC that filming was "about halfway through".
And she revealed that filming on the climactic battle for Hogwarts hasn't begun.
"The big stuff is still to come," she said. "I like that because if it's all at the start it's a bit of an anti-climax, but they're keeping that bit until the end."