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Film industry workers were treated to 24 minutes of footage from the movie about a battle on a distant planet.
Despite a media ban reporting on the clips shown, people at the event posted anonymously online that the footage was "jaw-dropping" and "stunning".
One Twitter user, who was at the event wrote: "This will change movies forever," while another said: "Footage from Avatar was stunning, literally jaw-dropping. Amazing visuals unlike any before seen, with incredible detail."
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Filmed using a mix of CGI, motion-capture animation and live action, Avatar is Cameron's first feature film project since 1997's Titanic.
Avatar is released in cinemas in the UK and US on 18 December.
* Pinched from BBC Entertainment News, 24/06/2009