Seth Rogen and James Franco stunned the star-studded audience at the MTV Movie Awards on Sunday by pretending to smoke marijuana onstage.

The pair stepped up to present an award for Best Summer movie and to promote their new stoner film Pineapple Express.

TV cameras at Universal City, California's Gibson Amphitheatre panned out as Rogen and Franco produced a bag of what appeared to be marijuana, and lit a joint.

While stars like Rihanna and former drug addict Robert Downey Jr. looked on stunned and uncomfortable, the pair smoked the cigarette and told the audience, "We're not pot heads at all. We don't smoke weed at all. This is not weed. And we're not really smoking a fat joint on live television at the moment."

They then called Downey Jr. onstage to collect the Award for movie Iron Man. The actor left question marks about the authenticity of the drugs, telling Rogen and Franco, "Thank you for that intoxicating introduction."