Director Spike Lee says we're "living in a very dangerous time" and that he hopes Donald Trump does not have the real nuclear codes.
He told a Cannes charity gala to pray false codes had been given to the US president instead.
Lee also revealed he refers to Mr Trump as "agent orange".
He told the Planned Parenthood event at the film festival's American Pavilion that he had "many happy memories in Cannes".
But he said of his failure to win the Palme d'Or in 1989 for nominated film Do the Right Thing: "Like Denzel, I got robbed."
Denzel Washington was nominated for the best actor Oscar this year - but missed out when Casey Affleck won.
He told the guests at the Wanderluxxe event benefitting Planned Parenthood to honour him and collaborator Roger Guenver Smith: "We're in the south of France, drinking French champagne. But we all go home and we're really going to face reality. It's no joke."
Lee, 60, said he hosted a benefit event for Barack Obama at his home, and saw the nuclear football in the president's car, where a member of staff was waiting.
Lee said: "I pointed to something inside and he nodded and it was the football. I thought it was a myth.
"You press the nuclear code and it can possibly end civilisation as we know it. I had nightmares that night."
Lee, a vocal opponent of Mr Trump, told the audience that "the guy has to be within 100 yards of the president" at all times.
He added: "This is no joke and every night I think about this, as agent orange has nuclear codes.
"There's two theories - he has the real code or the fake code.
"I pray to almighty God he has the fake one."
To laughter from those at the event, he asked them to make a prayer before going to bed.
"When your head hits that pillow, say a prayer for the universe that someone smart has given him the fake code to the football."