Superstar Beyonce and "Happy" creator Pharrell Williams won three prizes each on the music industry's biggest night, which was marked by displays of artist activism against domestic violence and police brutality.
The 22-year-old Smith swept up three of the night's four top trophies -- Record of the Year, Song of the Year and Best New Artist -- as well as Best Pop Vocal Album honors.
Smith, who not long ago was working as a bartender in London, thanked the man he fell in love with last year and who inspired "Stay With Me" -- his mega-hit ballad about a one-night stand.
"Thank you so much for breaking my heart because you got me four Grammys," Smith said to applause.
Smith has refrained from on-stage theatrics and has walked a fine line on his personal life, acknowledging his sexual orientation while striving not to be identified exclusively as a gay artist.
He earned an ovation at the Staples Center in Los Angeles when he performed his signature tune in his rich, soulful tenor voice with Mary J. Blige.
"Before I made this record, I was doing everything to try to get my music heard. I tried to lose weight and ... I was making awful music," said Smith.
"It was only until I started to be myself that the music started to flow and people started to listen. So, thank you, guys, for accepting me to be just me."
Smith was prevented from a clean sweep of the top categories by Beck, whose introspective "Morning Phase" -- full of lush orchestration around his acoustic guitar -- won the Album of the Year prize.
Beck, who also won Best Rock Album, has for two decades been known for his inventive and often ironic style which has won him critical acclaim and a cult following, if not the mainstream success usually recognized by the Grammys.
- 'Black Lives Matter' -
Beck's Album of the Year award was presented by the often-elusive Prince who, sporting a futuristic orange suit and carrying a staff, pointedly spoke out against a series of killings of African Americans that has galvanized public opinion.
"Like books and black lives, albums still matter," Prince said.
The theme was echoed throughout the Grammys.
The rapper Common referred to the tensions with police in Ferguson, Missouri, as he joined John Legend on the song "Glory," which is featured in the Oscar-nominated civil rights drama "Selma."
And Williams turned "Happy" -- normally a straightforward ode to joy -- into a more political track.
With his back-up dancers wearing hoodies, Williams led them in a "Hands Up, Don't Shoot" gesture -- a subtle protest against police shootings of African Americans.
US President Barack Obama also appeared unexpectedly on an overhead screen to urge action to stop violence against women.
Saying that more than one in four women has experienced some form of domestic violence, Obama declared: "It's not okay. It has to stop."
Survivor-turned-activist Brooke Axtell told the audience how she let a seemingly "handsome, charismatic man" abuse her, before Katy Perry sang an unadorned rendition of "By The Grace of God."
In a less grand gesture, rapper Kanye West defiantly raised his middle finger to the world as he performed with Paul McCartney and Rihanna.
And in another surprise, West said Beck did not deserve his Best Album win and that it should have gone to Beyonce instead.
"Beck needs to respect artistry and he should have given his award to Beyonce," he told E! television.
Madonna -- who was furious last month when songs from her upcoming album leaked ?- performed her new song "Living for Love" in a red matador-inspired outfit with half-naked male dancers in bull horns.
The music industry's top names opened the night by wearing red devil horns as aging hard rockers AC/DC performed two songs including "Highway to Hell" with drummer Chris Slade taking over from Phil Rudd, who is facing charges in New Zealand including making death threats.
- Another feat for Beyonce -
Beyonce won for Best R&B Song and Best R&B Performance with "Drunk In Love," her tale of marital bliss sung with her husband Jay-Z, as well as for the more technical Best Surround Sound Album.
With the latest honor, Beyonce has won 20 Grammys -- topping Aretha Franklin for the woman with the second biggest all-time haul, although still well behind the country star Alison Krauss at 27.
"This has been such an incredible year. I love you all. I'd like to thank my beloved husband. I love you deep," said the 33-year-old singer.
Williams took home prizes for Best Pop Solo Performance and Best Video for "Happy," as well as Best Urban Contemporary Album honors for "Girl."
Rosanne Cash -- the daughter of folk legend Johnny Cash -- took home awards in three Americana categories in her first Grammys since 1985.
Eminem won a record sixth Grammy for Best Rap Album for "The Marshall Mathers LP 2."
Iggy Azalea, a white Australian who has become a cultural flashpoint for performing in an African American accent, was one of the night's big losers.
