Daniel Radcliffe, Rupert Grint and Emma Watson were all at Leicester Square and were met by fans who'd been waiting for hours to see them.
Loads of the other actors from the film were there too, as was Harry Potter author JK Rowling.
The third film in the series is in cinemas on Monday 31 May.
Olivia Dixon, 15, from Hereford, made the journey to London in the hope that she would catch sight of one of the film's stars.
June Lovitt, nine, from Torquay, said she had heard the film was one of the scariest yet, but will still be going to see it on Monday.
"All my friends are talking about it. My favourite is Hermione, she's really brave and nice."
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