US actor Rami Malek says playing Queen lead singer Freddie Mercury in the film "Bohemian Rhapsody," which is finally ready for release after nearly a decade in development, was the greatest challenge of his career.

"It's the most challenging thing, I think, I could have ever endeavoured upon," Malek, 37, told dpa ahead of the film's London premiere.

"There are a lot of facets that people don't really know about Freddie Mercury and so that made it less daunting," he added.

Malek said his portrayal of Mercury, who died in 1991, aimed to "give people a window into an intimate part of his life - the young kid growing up in a foreign land, only reaching London at age 18, conflicted in so many ways with his identity, with his sexual identity, and how that manifested itself on stage."

"I was very cognisant of not wanting to mimic or imitate. Obviously, you have to bring a certain likeness to the person you're portraying...

"I'm trying to tell Freddie's story but it's a universal story as well," he said.

As a non-musician, capturing Mercury's powerful vocals and flamboyant stage performance also proved tough for Malek.

"I never considered myself a singer, I'd never touched a piano and I told everyone this from the very start, so it was an uphill climb for me," Malek said.

"I had to take lessons in everything, but I enjoy sitting at the piano now, quite a bit."

Malek has nothing but praise for the man whose life and music he studied for around a year while making the film.

"I think he's a revolutionary. I think he stood by authenticity, inclusivity and just not being a poster boy, not being marginalized, segregated," he said.

"Individuality was everything, and enjoying everyone else's individuality meant so much to him as well."

"Bohemian Rhapsody" takes the name of the 1975 Queen single whose blend of glam rock and opera caused a musical sensation in Britain and catapulted the already popular band to global pop stardom.

The film opens in Britain this week and in the United States from November 2.