Robert Downey Jr, Scarlett Johansson and Mark Ruffalo have joined forces for a star-studded ad that takes aim at Donald Trump.

Called Save the Day, Joss Whedon's hilarious ad manages to mock celebrity aid videos while asking Americans to vote in the upcoming general election.

The celebs, including Don Cheadle, Keegan-Michael Key, James Franco and Julianne Moore, call Trump a "racist abusive coward who could permanently damage the fabric of our society".

They ask: "Do we really want to give nuclear weapons to a man whose signature move is firing things?"

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They ask Americans to register to vote "to protect this country from fear and ignorance".

If they do, they promise Ruffalo will get nude in his next movie - which seems to come as something of a surprise to the Hulk star.

Watch the video below (warning: explicit language):

America's presidential elections take place on November 8. For more information, visit www.savetheday.vote.