Basketball legend Michael Jordan says he is "deeply saddened, truly pained and plain angry" after the death in police custody of African-American George Floyd in Minneapolis.
Jordan is one of a number of sports stars adding their voice to worldwide protests against racism.
"I see and feel everyone's pain, outrage and frustration," he added.
"I stand with those calling out the ingrained racism and violence toward people of colour in our country."
Jordan added: "We have had enough."
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Protests have been held after Floyd, an unarmed black man, died on 25 May after being restrained by white Minneapolis police officer Derek Chauvin, who kneeled on his neck for almost nine minutes to pin him down.
Chauvin has since been charged with his murder and sacked.
Vanessa Bryant, widow of basketball legend Kobe, shared an image of her late husband wearing an "I Can't Breathe" T-shirt.
"My husband wore this shirt years ago and yet here we are again," she wrote.
"Life is so fragile. Life is so unpredictable. Life is too short."
'We matter' – Rashford
Manchester United and England footballer Marcus Rashford said he had been "trying to process what is going on in the world".
He added: "At a time I've been asking people to come together, work together and be united, we appear to be more divided than ever.
"People are hurting and people need answers. Black lives matter. Black culture matters. Black communities matter. We matter."
British 200m world champion sprinter Dina Asher-Smith had tweeted on Thursday: "Racism, police brutality… all of this is something we all have to be vocal about. Irrespective of our race or nationality.
"RIP George Floyd. Heartbreaking and sickening to have to be saying RIP to another black person in these circumstances."