Monos,” an unsettling thriller about teenage soldiers that one critic described as “like ‘Lord of the Flies’ in hell,” has won the award for best film at the BFI London Film Festival.

Mati Diop’s “Atlantics,” which premiered in Cannes, took home the prize for best debut feature, while “White Riot,” about a 1970s reggae protest movement, was named best documentary. The awards were unveiled Saturday, on the eve of the festival’s closing.

Directed by Alejandro Landes, “Monos” premiered at Sundance. To an evocative score by Mica Levi, the Spanish-language film follows a group of teenage soldiers and their hostage, an American doctor, as they retreat farther into the jungle.

“‘Monos’ is a stunning cinematic achievement, marrying dynamic visuals, faultless performances and groundbreaking storytelling,” director Wash Westmoreland (“Colette”), the president of the official competition jury, said. “It’s a masterpiece.”

The jury also awarded special commendations to Sundance title “Honey Boy” by Alma Har’el and “Saint Maud” by first-time feature director Rose Glass, whom Westmoreland called “a powerful new voice in British cinema.” Those two films were among the six pictures directed or co-directed by women in the main competition lineup of 10 contenders. Among the other movies in the lineup were “Moffie,” “La Llorona” and “The Other Lamb.”

“Atlantics” chronicles the story of a young woman getting ready to marry another man after her lover leaves. The film competed for the Palme d’Or in Cannes.

“‘Atlantics’ is a film that intrigued us by its original and refreshing use of genre elements in a story that also has a strong political impact,” said director Jessica Hausner (“Little Joe”), who headed the jury for the Sutherland Award for best first feature. She described the film as “a crime scene that becomes a nightmarish tale, held together by a story of the endurance and persistence of young love.”

“White Riot,” winner of the Grierson Award for best documentary, takes a look at the Rock Against Racism movement that began in 1976. Directed by Rubika Shah, the film features interviews with and previously unseen footage of The Clash, Steel Pulse and X-Ray Spex.

Soheil Amirsharifi’s “Fault Line” (“Gosal”), about a courageous Iranian schoolgirl, won the short film competition.

“Our awards highlight the most distinctive, urgent and accomplished filmmaking from around the globe,” said LFF director Tricia Tuttle, “and it has been an incredible festival – with audiences moved, provoked and dazzled by these films, many of which engage with pressing social and political themes in very inventive ways.”

The 63rd London Film Festival closes Sunday with a screening of Martin Scorsese’s new gangster film, “The Irishman.”