The film about racial conflict in Los Angeles was the popular choice in a Harris poll of U.S. filmgoers, but was considered a dark horse behind the critic's choice, "Brokeback Mountain."
"Brokeback" also won three awards -- best director for Ang Lee, best score for Gustavo Santaolalla and best adapted screenplay for Larry McMurtry and Diana Ossana -- during the 78th Academy Awards ceremony at Hollywood's Kodak Theatre.
In accepting his award, Lee said "Brokeback" wasn't just about just gay love, "but the importance of love itself."
The best actor and actress awards reflected the Golden Globes, with Reese Witherspoon taking the Oscar for her portrayal of June Carter Cash in "Walk the Line" and Philip Seymour Hoffman being crowned best actor for "Capote."
George Clooney, who was nominated in the directorial as well as original screenplay categories for "Good Night, and Good Luck," too the best supporting actor trophy for "Syriana." The best supporting actress was Britain's Rachel Weisz for "The Constant Gardener."
"Memoirs of a Geisha" won Oscars for costume design, cinematography and art direction. "The Chronicles of Narnia: The Lion, the Witch and the Wardrobe" won for makeup, and "King Kong" took Oscars for visual effects, sound mixing and sound editing.
The most excited winners of the evening were the members of the hip-hop group, Three 6 Mafia, who won best original song for "It's Hard Out Here for a Pimp" from "Hustle & Flow."
The British team of Nick Park and Steve Box won the best animated feature award for "Wallace & Gromit in the Curse of the Were-Rabbit."
"March of the Penguins" was best feature documentary and South Africa's "Totsi" won the best foreign-language film award.
Politics was the informal theme at the Oscars. The host -- Comedy Central's fake news anchor Jon Stewart delivered an opening monologue similar to what he delivers nightly as host of "The Daily Show," mixing politics with satire and some gentle jabs at the celebrity audience.
Noting the nominated films include themes such as racism, terrorism and censorship, Stewart noted: "It's why we go to the movies, to escape."