Big names including Robert Downey Jr, Julianne Moore and Neil Patrick Harris have appeared in an ad attacking Donald Trump.

The ad was masterminded by Avengers director and Buffy the Vampire Slayer creator Joss Whedon, drawing on a huge pool of A-listers he has worked with across various projects.

In the clip, the actors – including Mark Ruffalo, Scarlett Johansson, Martin Sheen, Nathan Fillion and Cobie Smulders – attempt to emphasise the importance of the upcoming election.

They say: “You only get this many famous people together if the issue is one that truly matters to all of us.

“A disease or an ecological crisis or a racist abusive coward who could permanently damage the fabric of our society. Do the math.

“Do we really want to give nuclear weapons to a man whose signature move is firing things? We can end this nightmare before it begins. We can save the day.

“All it takes is all of us. We cannot pretend both sides are equally unfavourable. We cannot say one vote doesn’t matter. Your vote matters.”

However, it descends into hilarity when the stars promise Mark Ruffalo will do a nude scene in his next film if Trump is stopped.

Robert Downey Jr explains: “Mark’s going to have his dick out.”

“Full Monty”, an impassioned Neil Patrick Harris adds.

Whedon explained to Buzzfeed: “It’s about targeting people who either aren’t going to vote or have been anaesthetised out of voting – fighting the sort of apathy and cynicism that says, ‘It doesn’t matter if I vote,’.

There is almost nobody that I wouldn’t approach to say, ‘If you can pitch in, do it now.’

“It was pretty much the same spiel to everybody: ‘Doing a voting PSA to help get out the vote and stop orange Muppet Hitler.’”