LONDON (Reuters) - The BFI London Film Festival kicks off 12 days of movie premieres, screenings and talks on Wednesday with organizers this year focusing on promoting diversity.

"A United Kingdom", based on a true story about a British woman marrying an African prince in the 1940s and facing opposition, will start the proceedings when its stars Rosamund Pike and David Oyelowo walk the red carpet in Leicester Square later on Wednesday.

Oyelowo will also headline the festival's Black Star Symposium, which will look at the challenges black actors face in Britain and the United States, a topic in the spotlight after the outcry over the lack of diversity at this year's Academy Awards.

All of this year's acting Oscar nominees were white for a second consecutive year, leading to criticism with the social media hashtag #OscarsSoWhite. In June, the awards' organizers invited 683 new members to join, with a focus on female and minority talent.

"We need to see more films that reflect the society we actually live in," Oyelowo told Reuters in an interview to promote "A United Kingdom".

"The estimation of who we are, what we believe in as human beings is very much rooted in the films we see because they are supposedly a reflection of the lives we live and the societies we live in but I would argue that they are not."

Other movies showing at the festival include "Birth of a Nation" about an 1831 Virginia slave revolt and "Queen of Katwe", about Ugandan chess player Phiona Mutesi, in which Oyelowo also stars alongside Oscar winner Lupita Nyong'o.

Oliver Stone's "Snowden", about former National Security Agency contractor Edward Snowden, and fashion designer Tom Ford's "Nocturnal Animals" will also be screened at the festival which runs until Oct. 16.

"We have a very diverse audience and the stories we bring to the screen should be reflecting that audience," the BBC quoted festival director Clare Stewart as saying.

"12 Years a Slave" director Steve McQueen will be honored at the festival with the British Film Institute's highest accolade, the BFI Fellowship.

(Reporting By Marie-Louise Gumuchian; additional reporting by Jayson Mansaray Editing by Jeremy Gaunt.)