Director Lynne Ramsay's first feature film since 2002's Morvern Callar stars Tilda Swinton as a mother dealing with the fallout from her son Kevin's (Ezra Miller) horrific acts.
Filmmaker John Madden, who chaired the jury, said of the award: "This year's shortlist for 'Best Film' comprises work that is outstanding in terms of its originality and its stylistic reach. It is an international group, one united by a common sense of unflinching human enquiry and we were struck by the sheer panache displayed by these great storytellers.
"In the end, we were simply bowled over by one film, a sublime, uncompromising tale of the torment that can stand in the place of love. We Need to Talk About Kevin is made with the kind of singular vision that links great directors across all the traditions of cinema."
Watch the We Need to Talk About Kevin trailer below:
The 'Best British Newcomer' prize was awarded to Junkhearts actress Candese Reid, who jury chair Andy Harries described as "a fresh, brilliant and exciting new talent".
The 'Sutherland Award', handed out to the most original and imaginative filmmaking debut, was given to Las Acacias director Pablo Giorgelli.
Werner Herzog's Into the Abyss took home the 'Grierson Award for Best Documentary'. The acclaimed Austrian director's latest film examines the life on a man condemned to death in a Texas prison.
The prestigious BFI Fellowship went to Ralph Fiennes and David Cronenberg.
"The BFI London Film Festival Awards pay tribute to outstanding film talent, so we are delighted and honoured that both Ralph Fiennes, one of the world's finest and most respected actors and David Cronenberg, one of the most original and ground-breaking film directors of contemporary cinema, have both accepted BFI Fellowships - the highest accolade the BFI can bestow," said BFI chair Greg Dyke.
Previous recipients include Judi Dench, Danny Boyle, Clint Eastwood, Martin Scorsese, Ridley Scott and Orson Welles.
The BFI London Film Festival closes tonight with the premiere of Rachel Weisz's The Deep Blue Sea.
> BFI London Film Festival complete coverage
Watch Digital Spy's interview with We Need to Talk About Kevin director Lynne Ramsay and star Ezra Miller below: