Rhys Ifans has said it would be a "total disaster" for the world if Donald Trump became US president.
The Welsh actor and musician described the controversy surrounding the Republican candidate as "unbelievable", and said if he won it would be to the detriment of various people including NSA whistleblower Edward Snowden.
Rhys, 49, appears in Oliver Stone's new film Snowden, which tells the story of how the former NSA analyst leaked details of mass government surveillance in 2013.
Speaking at a screening of the movie at the London Film Festival, Rhys was asked if he thought a Trump triumph would be bad news for Mr Snowden who remains in exile in Russia.
"Well I think a Trump presidency, period, will mean a total disaster, not just for Edward Snowden, for women, for ethnic minorities, and for the world at large," he said.
Asked about his reaction to the controversy surrounding Mr Trump whose presidential campaign has been rocked by the leak of a tape in which he bragged about his fame allowing him to "do anything" to women, Rhys said: "It's very, very, very sad that we are having a discussion about things that are said which would appal me anyway, but the fact that it's someone who may be president of the largest super power in the world is kind of unbelievable."
Director Stone said Mr Snowden is a "historic figure", adding: "He will be remembered. The candidates are not talking about him. That means, I think, that he's doing something that's... It's not debatable. It's very hard to get this out to the public. He really believes in democracy, he thinks we should have one, but we don't vote on these issues. Let's do something about it."
Asked about the prospect of Mr Trump becoming president, he said: "I don't think he has a chance to win. I'd be worried about the continuation of the system that we have with Ms (Hillary) Clinton."
Labour leader Jeremy Corbyn posed for photos with Stone on the red carpet ahead of the screening in Leicester Square.