LOS ANGELES (AFP) - A furious Frank Stallone slammed the Academy in a foul-mouthed tirade Monday for snubbing big brother Sylvester in the best supporting actor category at this year's Oscars.

The Rocky star was widely backed to bag the prize following a Golden Globe award for his critically acclaimed turn as the boxer in Ryan Coogler's spin-off movie Creed.

But the statuette went instead to Britain's Mark Rylance for Bridge of Spies, in one of the biggest shocks of Sunday's gala ceremony at Hollywood's Dolby Theatre.

"The Academy should be ashamed of themselves. It's as clear as the nose on your face that Sly won. Mark who? It's total Hollywood bullshit," tweeted livid actor and musician Frank, 65.

Stallone himself was in more magnanimous mood, posting a picture of himself on Instagram in his black and blue Oscar tuxedo winking at the camera, fist clenched in defiance.

"To all the 'real Rocky's' of the world, Please hang on to your dreams, NEVER GIVE IN , NEVER GIVE OUT , NEVER GIVE UP! thanks for the support," said the 69-year-old, who was last Oscar-nominated 39 years ago as best lead actor and writer for Rocky.

A host of celebs rushed to ally themselves with Team Sly, including British media personality Piers Morgan, US actress Mia Farrow and Arnold Schwarzenegger, Stallone's co-star in action franchise The Expendables.

"Sly, just remember, no matter what they say, to me you are the best, you were the winner. I'm proud of you," Arnie said on a video posted to Facebook.

Rylance, 56, was asked backstage how it felt to have outpunched Stallone but said he didn't take awards too seriously.

"I don't know. People come up and say things about competing as actors and I know it's necessary to make a show out of it," he told reporters. "I feel more I am a spokesman than someone better than the (other) nominees."

Repeat telecasts of the 88th Annual Academy Awards will air on HBO (StarHub TV Channel 601) on March 2 (9pm), March 4 (10.30pm) and March 6 (4pm).