Famed US documentary maker Michael Moore used his win of an Oscar for his anti-gun documentary "Bowling For Columbine" to launch a violent attack on US President George W. Bush and the war, drawing loud boos from the audience.
"We live in a time with fictitious election results that elect fictitious presidents," he said referring to Bush's 2000 disputed election victory as he picked up the award for best documentary.
"We live in a time when we have a man sending us to war for fictitious reasons.
"We are against this war Mr Bush. Shame on you. Shame on you!," he said in front of a loudly booing audience of 3,500 people including most of Hollywood's top stars.
While other stars also made references to the war, which many Hollywood personalities oppose, Moore's protest was by far the most vitriolic as the war encroached further into cinema's top awards.
Welsh-born Catherine Zeta-Jones snatched the best supporting actress for her role as a murderous songstress, adding to four others taken by "Chicago" for best costume design, art direction, best sound and best cinematography.
"My hormones are too way out of control to be dealing with this," she said laughing.
"Chicago," the tale of greed, murder and jazz in the Windy City in the 1920s is nominated for 13 Oscars and appeared to be heading for a definitive victory.
Its competition included the drama "The Hours," with nine nods, Holocaust story "The Pianist" with seven 10-times nominated epic "Gangs of New York."
Chris Cooper won the best supporting actor Oscar for his gritty best supporting actor role as horticulturalist John Laroche in the dark comedy "Adaptation."
He became the first celebrity to speak up for peace as he accepted his statuette -- to thunderous applause from an audience as concerned by the war in the Middle East as by the Oscars.
"In light of all the troubles in this world I wish us all peace," he said, to a major round of applause.
And one of the Oscar presenters Gael Garcia Bernal, who starred in the nominated movie "Frida," broke the Oscars' cardinal rule this year by diverging from his script and speaking out against the war.
"If Frida were alive she would be on our side against the war," he said of Mexican artist Frida Kahlo on whom the film starring Salama Hayek was based.
The Oscars telecast was interrupted twice in the first two hours to bring live news of the bombing campaign in Iraq, while host Steve Martin began the evening by quipping about the lack of glitz at the toned-down red carpet arrivals ceremony.
Hollywood's biggest night got underway after days of nail biting over whether war would derail the show, with stars turning out in force and in style, many bearing messages of peace.
In the days preceding the ceremony, organizers feared that top Tinseltown draws would boycott cinema's top honours due to concerns they would appear insensitive in celebrating their industry while war is raging in Iraq.
Those fears proved to be unfounded although some celebrities did pull out of the awards, including actors Will Smith, Angelina Jolie, Cate Blanchett, "Lord of the Rings" director Peter Jackson -- whose film picked up the best visual effects gong.
While glamour and style were everywhere at the scaled down arrivals ceremony -- drastically cut back as a nod to the war -- the fashion tone was muted compared to other years.
But war themes were ever-present. Anti-war activist and Oscar presenter Susan Sarandon and her husband Tim Robbins flashed peace signs as they arrived, as did several other celebrities.
Other celebrities wore a blue lapel badge bearing the international symbol for peace including Julianne Moore directors Todd Haynes and Rob Marshall and Spanish director Pedro Almodovar wore a dove of peace on his lapel.
Actor Andy Serkis, who played Gollum in "Lord of the Rings" meanwhile walked down the reduced red carpet brandishing a red and yellow sign saying: "No war for oil."
While Renee Zellweger is favourite for best actress for "Chicago", she goes head-to-head with new Hollywood force Nicole Kidman -- nominated for her role as Virginia Woolf in "The Hours".
Daniel Day-Lewis was strongly favoured for best actor for his role as villainous Bill the Butcher in "Gangs," but in an upset he lost out to Adrien Brody's role as a pianist struggling to survive the Holocaust in "The Pianist."
Brody, 29, is the youngest-ever best actor winner.