Actor Will Smith hit comedian Chris Rock and verbally abused him during the Oscars presentation after the comedian made a joke about his wife Jada Pinkett Smith's shaved head.
Smith then went on to win the award for Best Actor and apologised to the Academy and his fellow nominees in his acceptance speech, but not to Rock.
Los Angeles police said Rock declined to file a police report but they were prepared to investigate later, if asked.
Rock was presenting an award when he made a joke about Smith's wife appearing in a GI Jane sequel.
"Jada, I love you. GI Jane 2 — can't wait to see it, alright," Rock said.
GI Jane is a 1997 movie featuring Demi Moore, who played a Navy SEAL and shaved her head for the role.
Pinkett Smith, who has hair loss, looked visibly annoyed at Rock's comment when the camera cut to her reaction.
The live coverage then moved back to Rock, who brushed off the reaction, saying "that was a nice one".
Moments later, Smith walked onto the stage and hit Rock in the face with what appeared to be an open palm.
It was initially unclear if the moment was scripted, as Rock smiled to say: "Oh wow, wow, Will Smith just smacked the s*** out of me".
But then Smith, who had returned to his seat, shouted to Rock, who was still on stage.
Smith: "Keep my wife's name out of your f***ing mouth."
Rock: "Wow dude, it was a GI Jane joke."
Smith (louder): "Keep my wife's name out of your f***ing mouth."
Rock: "I'm going to, OK … That was, ugh … the greatest night in the history of television. OK … OK."
The incident was not censored when it was broadcast in Australia.
Will Smith calls himself 'protector' in Oscar speech
Smith remained in his seat next to Pinkett Smith after the incident and Rock continued to present the award for Best Documentary.
Many in the audience, and those watching at home, were stunned by the assault and were unsure if it was real as the Oscars includes scripted moments for "entertainment".
However, American rapper Diddy, the presenter after Rock, addressed it in his speech.
"Will and Chris, we're going to solve that like family. Right now, we're moving on with love. Everybody make some noise," Diddy said.
Actors Denzel Washington and Tyler Perry, and Smith's publicist Meredith O'Sullivan, spoke to Smith during the commercial break.
Smith went on to win his first Oscar later in the evening.
He received the Best Actor award for his role as Richard Williams, the father of tennis champions Serena and Venus Williams, in King Richard, which was also nominated for Best Picture.
In his acceptance speech, the 53-year-old actor indirectly acknowledged the confrontation.
"Richard Williams was a fierce defender of his family," Smith said.
"In this time in my life, in this moment, I am overwhelmed by what God is calling on me to do and be in this world.
"I am being called on in my life to love people and to protect people, and to be a river to my people. I know to do what we do, you gotta be able to take abuse and you gotta be able to have people talk crazy about you.
"In this business, you've gotta have people disrespecting you, and you've got to smile and pretend that's OK.
"Denzel said to me a few minutes ago, 'At your highest moment, be careful, that's when the devil comes for you'.
"I want to be a vessel for love. I want to say thank you to Venus and Serena and the entire Williams family for entrusting me with your story. That's what I want to do. I want to be an ambassador for that kind of love, and care and concern.
"I want to apologise to the Academy, I want to apologise to all my fellow nominees.
"Art imitates life: I look like the crazy father, just like they said about Richard Williams. But love will make you do crazy things.
"Hoping the Academy invites me back."
In a statement released to local media, LA police said it was aware of an incident at the Oscars, but did not name Smith or Rock.
"LAPD investigative entities are aware of an incident between two individuals during the Academy Awards program," the statement said.
"The incident involved one individual slapping another.
"The individual involved has declined to file a police report. If the involved party desires a police report at a later date, LAPD will be available to complete an investigative report."
Jada Pinkett Smith open about living with alopecia
Pinkett Smith has spoken publicly about living with a form of alopecia, an autoimmune condition that causes hair loss.
The 50-year-old actress first revealed her diagnosis in 2018.
In December 2021, she shared a video on Instagram explaining she had shaved her head because she could not hide the drastic changes to her hair.
She has attended several red carpets, including the Oscars on Sunday night (local time), with her head bare.
Rock has hosted the Oscars twice, in 2005, when he received mixed reviews, and again in 2016 when, during the #OscarsSoWhite protests, he confronted the Academy over the lack of black representation.
In 2016, Pinkett Smith suggested on Twitter that people of colour boycott the awards, and did not attend with her husband.
In his opening speech, Rock said: "Jada boycotting is like me boycotting Rihanna's panties … I wasn't invited."
