Activists chose orange -- the color won by hunters to alert others to their presence -- as the official hue of the event, symbolizing the value of human life.
The first-of-a-kind National Gun Violence Awareness Day marked what would have been the 18th birthday of Hadiya Pendleton, a girl shot dead in a Chicago park in 2013 days after performing in a parade in Washington as part of President Barack Obama's second inauguration.
"I think the fact that our heart hurts so much, that's the reason we're motivated to get into a movement," her mother, Cleopatra Cowley-Pendleton, said in a video.
Mayors of major US cities wore orange including Bill de Blasio of New York, Eric Garcetti of Los Angeles and Rahm Emanuel of Chicago.
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The New York Mets released pictures of the Major League Baseball team as well as the staff wearing orange as part of the day.
Other prominent figures who joined the campaign included Oscar-winning actress Julianne Moore, REM singer Michael Stipe, punk legend Patti Smith and the full roster and staff of Motown Records.
MTV also turned its logo orange to raise awareness about gun violence.
The 2012 massacre of 20 children and six adults at the Sandy Hook Elementary School in Connecticut brought renewed calls for action to stem gun violence.
But measures backed by Obama such as stricter background checks for gun purchases ultimately failed amid fierce opposition by firearms enthusiasts.
Around 10 Americans out of every 100,000 die each year from gun-related homicides or suicides, a rate far higher than in any other major industrialized country.