By Valerie Hamilton

Los Angeles (dpa) - The journalism drama Spotlight won the Oscar for best picture Sunday in an unusually serious Academy Awards ceremony that was upended by a public debate over racism in Hollywood.

The film chronicles the true story of a team of investigative reporters who exposed a Catholic church cover-up of sexual abuse by priests in Boston.

"This film gave a voice to survivors," producer Michael Sugar said, accepting the award, and added that he hoped "that voice will become a choir that will resonate all the way to the Vatican."

"Pope Francis, it's time to protect the children and restore the faith," he said.

The film's win was a sombre finale to an Academy Awards ceremony coloured by serious social issues, first among them the debate over race that has overshadowed the film world's annual awards gala for weeks.

Oscars host Chris Rock kicked off the Oscars telecast Sunday by asking a taboo question - and answering it.

"Is Hollywood racist? You're damn right Hollywood is racist," the African-American comedian said, addressing Hollywood's top stars assembled in the Dolby Theatre.

The presentation was a stark contrast to the ceremony's traditional focus: a celebration of the year's best cinematic achievements.

The dystopian action movie Mad Max: Fury Road was the evening's top winner by numbers, taking Oscars in six categories in what was nearly a clean sweep of the technical awards.

The Revenant, Alejandro Gonzalez Inarritu's snowbound survival epic, won three, for directing, cinematography and best actor (Leonardo DiCaprio).

DiCaprio had been nominated six times for the Oscar since 1994, but this was his first win, and the audience celebrated it with raucous cheering and a standing ovation.

Accepting the award, DiCaprio delivered a thoughtful speech on climate change, calling it "the most urgent threat facing our entire species."

Room star Brie Larson won best actress for her performance as a kidnapped mother introducing her son to the world outside. She had been a heavy favourite for the award after winning best actress at every major awards this season.

Swedish actress Alicia Vikander won best supporting actress for her work in The Danish Girl, while in an upset, British stage actor Mark Rylance won best supporting actor, a prize that had been widely expected to go to sentimental favourite Sylvester Stallone.

Rylance won for his portrayal of real-life Soviet spy Rudolf Abel in Steven Spielberg's Cold War thriller Bridge of Spies.

In addition to best picture, Spotlight won the prize for best original screenplay, while The Big Short won for best adapted screenplay.

Best documentary feature went to Amy, a story of the life and death of British singer Amy Winehouse. Pakistani filmmaker Sharmeen Obaid Chinoy won for best documentary short subject for her film about "honour killings," A Girl in the River: The Price of Forgiveness.

Hungarian Holocaust drama Son of Saul won for best foreign film, while the Pixar coming-of-age story Inside Out won for best animated feature.

But the focus of the evening was as much on those who were not part of the celebration as those who were.

A public debate about racial bias in Hollywood erupted in January after few non-Caucasian artists were nominated for the awards, and none at all for acting prizes, for the second year running.

Rock, known for his frank talk about race, made the race debate the theme of the show, from his 10-minute opening monologue to pointed taped segments challenging the marginalization of non-white artists.

In one segment played during the show, Rock gave Oscars to black movie-goers and asked for their acceptance speeches.

"There's talent out there of all races," said one man, to applause from the live audience.

The broadcast touched on other serious themes.

US Vice President Joe Biden teamed up with Lady Gaga to call for action on sexual violence. Lady Gaga performed her song 'Til It Happens To You joined onstage by dozens of sexual assault survivors.

Later in a tweet, US singer Kesha thanked Biden and Lady Gaga for their support for victims of sexual assault, saying in a tweet: "It hit very close to my heart for obvious reasons."

Kesha is in the midst of a legal battle in which she is accusing her music producer Dr Luke of years of manipulation and abuse.

British singer Sam Smith, accepting the Oscar for best original song for Writing's On The Wall, dedicated his award to the lesbian, gay, bisexual and transgender community.

"I stand here tonight as a proud gay man and hope we can stand together as equals one day," he said.

The Oscars are presented by the Academy of Motion Picture Arts and Sciences. The Academy's approximately 6,200 voting members choose winners in 24 categories.