Collecting the best documentary award for his film Bowling for Columbine, Moore blasted: "We are against this war, Mr. Bush. Shame on you."
Wagging his finger from the stage as he drew a mix of applause and boos from assembled stars, he added: "We like nonfiction and we live in fictitious times.
"We live in a time where we have fictitious election results, elect a fictitious president. We live in a time where we have a man sending us to war for fictitious reasons."
Moore's speech was cut short as show producers drowned him out with music from the orchestra.