The documentary maker won his first Oscar for "Bowling for Columbine," but he brought the other nominees on stage with him in what he called a show of solidarity for nonfiction during these "fictitious times."
"We live in the time where we have fictitious election results that elect a fictitious president," Moore said. "We live in a time where we have a man who's sending us to war for fictitious reasons, whether it's the fiction of duct tape or the fiction of orange alerts.
Applause gave way to some boos, as the orchestra began playing to cue the filmmaker to leave the stage.
"We are against this war, Mr. Bush. Shame on you, Mr. Bush. Shame on you," Moore shouted.
Afterward, host Steve Martin tried to restore levity.
"It was so sweet backstage, you should have seen it," Martin joked. "The Teamsters were helping Michael Moore into the trunk of his limo."
"Bowling for Columbine" was Moore's exploration of gun violence in America. The title refers to the fact that gunmen Eric Harris and Dylan Klebold went bowling before they opened fire at Columbine High School in Colorado, killing 12 students and a teacher before turning the guns on themselves.
Asked backstage why he made the remarks, Moore answered: "I'm an American."
"Is that all?" a reporter asked.
"Oh, that's a lot," Moore responded.
He dismissed the jeers he received, telling reporters: "Don't report that there was a split decision in the hall because five loud people booed."
The rotund, scruffy-bearded activist from Flint, Mich., also directed the 1989 documentary "Roger & Me," in which he pursued former General Motors Corp. boss Roger Smith to confront him about the collapse of the auto industry in Moore's hometown.
He's also the author of the best-selling book "Stupid White Men ... And Other Sorry Excuses for the State of the Nation," which criticizes American politicians for favoring corporate wealth over public well-being.
Scattered appeals for peace and grim reports from the U.S.-led war in Iraq added a sober contrast to Hollywood's traditional night of glitzy self-glorification.
"In light of all the troubles in this world, I wish us all peace," Chris Cooper said during his acceptance speech for best supporting actor for "Adaptation."
Cooper was among several nominees, including Meryl Streep and Martin Scorsese, who wore dove peace pins on their formal wear as a silent statement about the war.
Susan Sarandon and Tim Robbins drew the loudest cheers when they flashed matching peace signs after exiting their gasoline-electric hybrid car on Hollywood Boulevard at Sunday's Oscars.
Director Pedro Almodovar, who has been outspoken against the U.S.-led war in Iraq, formed a peace sign in the direction of a crowd lined up 10-deep behind a chain-link fence covered with a black screen.
Sentiments about the war, however, came mostly from crowds gathered near the Kodak Theatre since there was a truncated red carpet arrival by the stars.
Oscar officials did away with bleachers to accommodate shouting fans and didn't allow the reporters and photographers who traditionally cram the carpet as the stars arrived.
"You probably noticed there was no fancy red carpet tonight," Oscar host Martin joked during his opening monologue. "That'll send them a message."
Along the boulevard, helicopters buzzed overhead as usual, but the street was oddly quiet with journalists kept at a distance from guests.
Fans stood mostly silent as everything from black limos to fuel-efficient hybrid cars to a green taxi and a white van dropped high-profile passengers in the middle of the street.
Only when recognizable faces such as Sean Connery, Julianne Moore, Hilary Swank and Nia Vardalos turned and waved did the crowd cheer. Moore blew a kiss to the fans.
Comedian Marty Ingels, husband of Oscar winner Shirley Jones, whipped open his black topcoat to reveal an American flag sewn inside.
Some in the crowd toted signs reading "No Iraq War" and "Bring U.S. soldiers home!"
Half a block away, crowds gathered behind metal barricades waved American flags. At times, their chants of "USA, Free Iraq" and "Liberate Iraq" could be heard within earshot of the celebrities.