12 Years a Slave has won the top prize of best film at the Baftas, seeing off competition from American Hustle and Gravity. It adds to its award for Chiwetel Ejiofor in the best actor category, and mirrors the Golden Globes in winning best film but losing the prize for best director.

"Roll Jordan Roll, that's for sure," said the film's director Steve McQueen, quoting the film's central song, and he first thanked his cinematographer Sean Bobbitt, raising a big cheer. Of his female star Lupita Nyong'o, nominated for best supporting actress, he said "a star is born". McQueen also called the film's co-star Michael Fassbender a "genius", and praised Ejiofor for bringing "the humanity in this film". To his mother, McQueen said: "Thank you for having the faith."

"There are 21 million people in slavery as we sit here," McQueen said, adding that he hoped that in 150 years a film like his wouldn't need to be made.

The film stars Ejiofor as a free man who is kidnapped and sold into slavery in antebellum US. It has been both a critical and commercial success for its unflinchingly violent and devastatingly moving portrayal of the realities of the slave trade.

The film goes to the Oscars on 2 March as the favourite for the best film prize. 12 Years a Slave is nominated for eight other Academy awards.

In a press conference following the win, McQueen celebrated his source material for the film, the memoir of the central figure, Solomon Northup: "Every turn of the page was a revelation. It was a treasure." He also praised Ejiofor for his best actor win, saying: "I'm so happy he's been recognised in this way. He's the only person who could have done this. It's an astonishing performance."

Following an informal meeting with McQueen after the release of Hunger, Ejiofor was sent the script for Slave and says he "was struck with self doubt" at the thought of playing the role thanks to the gravity of the story. But the source material turned him around. "Actually it was about understanding and connecting to Solomon Northrup… it was a way for me to tell the story and the rest could be revealed."

12 Years a Slave co-producer Brad Pitt said: "We're very proud of our work here; it means a lot to us because of the people we got to work with. It's a story that says we're all the same, that freedom and dignity is everything and that's what we have to fight for."

And to a man who asked whether he's bothered if people focus on his looks and what he's wearing at awards ceremonies, rather than the work, Pitt simply replied: "Fuck it."

• The winners as they are announced
• Xan Brooks is live-blogging the ceremony here