Author JK Rowling has admitted that Donald Trump's win left her in a "bleak mood" as she travelled to America to promote the Harry Potter prequel Fantastic Beasts And Where To Find Them.

The movie, which stars Eddie Redmayne and Colin Farrell, had its world premiere in New York on Thursday night.

At a press conference before the red carpet event, the author, who's been a vocal critic of Trump, refused to speak directly about the president-elect but revealed the story of Fantastic Beasts was inspired by the rise of "populism".

"I think today is a day to focus on some good things and putting some good things out into the world hopefully," Rowling said.

Rowling walking the red carpet at last night's World Premiere

"If you have read the Potter books, you'll know this period of history threatened to become very dystopian. You were looking at the rise of a very dark force. I conceived of this story a few years ago. It was partly informed by what I see as a rise of populism around the world."

She added: "Last night I was in a kind of a bleak mood boarding the plane and I thought I need to work. I got out the second screenplay and worked on that and it made me feel a whole lot better."

Rowling also said she was "delighted" Johnny Depp had been cast in the next instalment of Fantastic Beasts. The author said Depp, who faced domestic abuse accusations during his divorce battle with Amber Heard earlier this year, had done "incredible things" with his portrayal of dark wizard Grindelwald.

Depp, who vehemently denied the abuse allegations, appears briefly in Fantastic Beasts but is set for a more prominent role in the planned sequel.

When asked about his casting in the movie, Rowling said that "I'm delighted. He's done incredible things with that character."

Director David Yates said Depp filmed his brief sequence in Fantastic Beasts in January and urged Harry Potter fans to trust the filmmakers about his casting.

"You have to trust us and see what he does in this movie. You won't see very much in this movie because he appears in it very fleetingly. He appears much more in the second movie", he said.

"He's a huge Potter fan. He loves the world. He was beyond excited about working on this material. In fact he didn't even want to see a script, he just said 'I'm in'."

It has been reported that a sequel to Fantastic Beasts may feature a gay relationship between the character of Grindelwald and a young Professor Dumbledore from the original Harry Potter stories.

Addressing the speculation, Rowling said that she couldn't reveal everything "because obviously it's a five-part story and there's lots to unpack in that relationship"

"I will say you'll see Dumbledore as a younger man and quite a troubled man because he wasn't always the sage. He was always very clever. We'll see him at what I think was the formative period of his life. As far as his sexuality is concerned, watch this space."

Rowling recently revealed that the film will be the first of five movies in the spin-off Harry Potter series. The series was originally intended as a trilogy, but the author said that when devising the plot that she needed more time to tell the whole story.

Fantastic Beasts And Where To Find Them is released in Ireland on November 18.