Sean Penn's casual green card remark about Mexican director Alejandro Gonzalez Inarritu touched off a firestorm of outrage on social media following the Oscars.

Penn was on stage to announce the final Academy Award of the night, a Best Picture win for Inarritu's film 'Birdman.' The Mexican-born director had also just won top directing honours.

Penn opened the envelope with the Best Picture winner, paused for a moment, then said: "Who gave this son of a b--- his green card? 'Birdman.'"

It was a casual joke by an actor who once worked for Innaritu in '21 Grams,' but it didn't sit well with many who were watching the internationally-televised awards show.

Innaritu didn't appear bothered by the joke. Instead, he used part of his Best Picture acceptance speech to speak about the plight of Mexicans in the United States, and to remind his audience that the U.S. was formed by immigrants from other nations long ago.

It wasn't the only time racism reared its head during the telecast. Host Neil Patrick Harris opened the night by poking fun at the number of white people nominated this year.

"Today, we honour Hollywood's best and whitest. Sorry… brightest," he said.