A judge has ruled a white former St Louis police officer was not guilty of murder in the 2011 shooting of a black man, sending hundreds of protesters to the city's streets to voice their anger.
With the National Guard on standby in case of violence, authorities appealed to protesters to march peacefully in a state that has seen racially charged clashes in the past.
Jason Stockley, 36, was acquitted of first-degree murder for killing Anthony Lamar Smith, 24. The former policeman, who was arrested in May 2016, was accused of planting a gun in Smith's car but testified he acted in self-defence.
After the verdict, more than 500 protesters marched in downtown St. Louis, chanting "No justice, no peace" and "Shut it down." Some held "Black Lives Matter" signs.
"I'm sad, I'm hurt, I'm mad," the Reverend Clinton Stancil of the Wayman AME Church in St. Louis said by telephone. "We haven't made any progress since Ferguson, that's clear. Cops can still kill us with impunity."
Police said on Twitter that one person was taken into custody after damaging a police vehicle.
In recent years grand juries have declined to charge officers involved in the 2014 fatal shooting of Michael Brown, 18, which set of nights of violent protest in Ferguson, near St Louis, and the choking death of Eric Garner, 43, in New York. Baltimore police officers also were not convicted in the case of Freddie Gray, who died from a broken neck suffered in a police van in 2015.
Brown's father, Michael Brown Sr, voiced his frustration after Friday's verdict.
"You all know this ain't right and you all continue to do this to us," he told a St Louis Fox television station. "Like we don't mean nothing, like we're rats, trash, dogs in the streets. Right now, I'm praying for my city because my people are tired of this."