Newcomer Lupita Ngong'o has won the best supporting actress Oscar for her debut film role in 12 Years a Slave.
The Kenyan actress paid tribute to her character, slave worker Patsey, saluting her for her "guidance".
"So much joy in my life is thanks to so much pain in someone else's," said the 31-year-old, who celebrated her birthday this weekend.
It marked the first award of the night for 12 Years a Slave which is tipped to win best picture.
Based on a true story and directed by Briton Steve McQueen, it follows the life of Solomon Northup, a free black man in New York kidnapped and sold into slavery in Louisiana.
The historical drama began the evening with nine nominations, with Gravity and American Hustle each receiving 10.
Jared Leto won the first Oscar of the night, picking up best supporting actor for his role in Aids drama Dallas Buyers Club.
Leto, who played a transgender woman, dedicated his award to "those who have ever felt injustice because of who they are, or who you love".
In an emotional speech he thanked his mother "for teaching me to dream".
Leto - in his first film for six years - beat fellow nominees Jonah Hill, Bradley Cooper, Michael Fassbender and newcomer Abdi Barkhad, who won a Bafta last month for his role in Captain Phillips.
"This is for the 36 million people out there who have lost the battle to Aids," said Leto.
Dallas Buyers Club also picked up a second award for make up and hairstyling - with the transformation of Leto and his co-star, Matthew McConaughey, rumoured to have been achieved on a budget of $250 (£150).
Frozen, which recently tipped $1bn (£600m) at the global box office, was named best animated feature film ahead of Despicable Me 2, The Croods, The Wind Rises and Ernest & Celestine.
The 3D film about an icy princess and her sister was directed by Chris Buck and Jennifer Lee, with voicing by Idina Menzel, Kristen Bell and Josh Gad. It is loosely based on The Snow Queen by Hans Christian Andersen.
The first British win of the night came for Tim Webber and his team from London-based company Framestore for their visual effects work on Gravity.
Webber paid tribute to his team, Gravity actors George Clooney and Sandra Bullock and, above all, director Cuaron "for having the vision to create this breath-taking film and the audacity to make it happen".
The second British win of the night went to director Malcolm Clarke, who won an Oscar for his documentary short The Lady in Number Six: Music Saved My Life.
The film follows Alice Herz-Sommer, the oldest known survivor of the Nazi Holocaust and an accomplished pianist. Paying tribute to Herz-Sommer, who died last week at the age of 110, he praised "her extraordinary capacity for joy and amazing capacity for forgiveness".
"She taught everyone on my crew to be a little bit more optimistic," he added, dedicating his award to her.
Italy's The Great Beauty - La Grande Bellezza - won the best foreign language film, beating contenders from Denmark, Belgium, Cambodia and Palestine. It marked Italy's 11th win - the last was 1997's Life is Beautiful, which also won best actor for Roberto Benigni.
Catherine Martin picked up two Oscars, adding to her previous haul of two (for Moulin Rouge) - for costume design and production design on partner Baz Luhrmann's Great Gatsby adaptation.
In what is considered to be the strongest field for many years there are nine contenders for best picture.
British film Philomena, based on the true story of an Irish woman trying to find the son she was forced to give up for adoption, is up for four awards, including best picture and best actress for Dame Judi Dench.
Captain Phillips, 12 Years a Slave, Nebraska, The Wolf of Wall Street, Dallas Buyers Club and Her are also among the nine best picture nominees.
In the best director category, David O Russell faces competition from McQueen, Cuaron, Nebraska's Alexander Payne and Martin Scorsese for The Wolf of Wall Street.