The founder of the whistle-blowing website lashed out at the movie in a speech to the Oxford Union.
Holding a copy of the script, he singled out one part which suggested Iran is working on an atomic weapon.
He asked: ‘How does this have anything to do with us?
‘It is a lie upon lie. The movie is a massive propaganda attack on WikiLeaks and the character of my staff.’
His speech was delivered via video link from the Ecuadorian Embassy in London.
Benedict Cumberbatch will play Assange in the new film which is being made by Steven Spielberg’s Dreamworks company.
The first picture from the movie was released this week with Cumberbatch, showing off his new blonde hair, standing next to Inglorious Basterds star Daniel Bruhl, who has has been cast as Assange’s spokesman, Daniel Domscheit-Berg.
Downton Abbey’s Dan Stevens, Laura Linney, David Thewlis and Peter Capaldi also make up the cast.
The Fifth Estate is due out in the US on November 15, with a UK release date yet to be announced.