The Oscar race has been pronounced over, six months before the ceremony itself. At the Toronto film festival, the premiere of British director Steve McQueen's 12 Years a Slave was met on Friday evening with an overwhelming reception: gasps, sobs, a smattering of walk-outs at particularly brutal moments, and finally, a prolonged standing ovation.

The crowds leaving the auditorium were primed to place bets on the film being an unbeatable contender for best picture, as well as McQueen for best director, best actor for Chiwetel Ejiofor, best supporting actor for Michael Fassbender, best supporting actress for Lupita Nyong'o, as well as the full slate of technical nods.

12 Years a Slave met with ecstatic reviews when a sneak preview debuted at the boutique film festival in Telluride last week, but the raves coming out of Toronto are likely the crucial second step in what looks certain to be a triumphant awards campaign.

McQueen's third feature as director, following 2008's Hunger and 2011's Shame, 12 Years a Slave is very faithfully adapted from the memoir by Solomon Northup, a free man living with his family in relative affluence near New York, who in 1841 was duped, drugged, abducted and sold into slavery. John Ridley adapted the book for the big screen, McQueen's partner, Bianca Stigter - who he thanked on stage before the premiere - was the person who originally suggested it as a source.

Benedict Cumberbatch plays Solomon's first, more progressive owner; Michael Fassbender - McQueen's longterm collaborator - his much less benevolent second. Slave shares much of the aesthetic (particularly the unflinching violence) that distinguished McQueen's earlier films, yet here the splashy tech setpieces have been cast aside. This is a film in the service of both its story and a hero who's much more unequivocally sympathetic than those from Hunger and Shame. The odd flash of McQueen's installation-origins remains - a burnt piece of paper in the pitch black night, its embers dying like shrinking larvae - but this is also accessible and immediate; a winning mix of mainstream and arthouse.

After the final credits rolled, McQueen returned to the stage with his cast and crew, including Brad Pitt, who as well as producing through his Plan B label, also plays a sympathetic carpenter. Of the decision to back the project, Pitt said: "Steve was the first to ask the big question: why have there not been more films on American history of slavery? It took a Brit to ask it …. And I just have to say: if I never get to participate in a film again, this is it for me."

The premiere is likely also it for the festival, just one day in. If the notion that 12 Years a Slave won't win the best picture Oscar seems absurd to those who've seen it, the idea that it wouldn't take the audience award - the sole honour at the Toronto film festival - is plain insane.

• This article was amended on 7 September to explain the reason for the walkouts and name McQueen's partner.

• Full coverage: 12 Years a Slave