Magic Johnson, Chris Rock and LeBron James were among those tweeting after a night of unrest in Ferguson, Missouri linked to the death of aspiring college student Michael Brown, 18.
"The system enables young black men to be killed behind the mask of law," wrote basketball icon Kobe Bryant yesterday, while filmmaker Spike Lee posted a photo of the slain teen with the caption "We are Mike Brown."
"I am very disappointed with decision in Mike Brown case in Ferguson, MO," wrote Johnson. "We must work together to stop the unnecessary loss of young men of colour. Justice was not served in Ferguson."
Singer Katy Perry, on tour in Australia, tweeted: "Feeling stuck all the way over here in Sydney seeing the news just now... Sending my prayers to Ferguson & praying for an equal America."
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Cher said "Have problem re decision not 2bring 2 trial police who shot young man near St Louis," but added: "Having said that, violence is NOT the ANSWER. I vote 2change state laws. There must be change in attitude and BALANCE in police and lawmakers."
Black US comedian Rock quoted civil rights activist W E B Du Bois, saying: "A system cannot fail those it was never meant to protect."
"Girls" creator and actress Lena Dunham tweeted love to Brown's family, adding: "I applaud their strength and courage, mourn their loss, pray they see justice and change."
Basketball player James posted a picture of Brown walking arm in arm with Trayvon Martin, another black youth controversially killed.
"As a society how do we do better and stop things like this happening time after time!! I'm so sorry," he wrote.