(CNN) -- Just steps away from the courthouse where a jury decided George Zimmerman's fate, demonstrators vowed that their fight wasn't over. "Nationwide protest to demand justice," they chanted after the jury's not guilty verdict in Sanford, Florida, cleared Zimmerman in the shooting death of Trayvon Martin. It wasn't long before some appeared to be heeding their call. Protesters rallied in Los Angeles, San Francisco, Chicago, Denver, Baltimore, Detroit, New York and other cities. "I came here today to stand in solidarity with Trayvon Martin's family, and everyone else searching for justice for him, and wanting to bring Zimmerman to justice," said Kellie Knight. She spoke at a protest at Union Square in New York City.
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NEW: Protesters rally in New York, Denver, Baltimore and Detroit
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NEW: "If it can happen to her son, it can happen to anybody's son or daughter," says one
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Obama urges "calm reflection," asks Americans to reflect on toll of gun violence
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Leaders call for protests to remain peaceful, push for change
Any act of violence could serve to undermine the innocent blood and moral authority of @placeholder.