ST. LOUIS - A white former police officer was acquitted Friday in the 2011 death of a black man who was fatally shot following a high-speed chase, and hundreds of demonstrators streamed into the streets of downtown St. Louis to protest the verdict that had stirred fears of civil unrest for weeks.

Ahead of the acquittal, activists had threatened civil disobedience if Jason Stockley were not convicted, including possible efforts to shut down highways. Barricades went up last month around police headquarters, the courthouse where the trial was held and other potential protest sites. Protesters were on the march within hours of the decision.

The judge who decided the matter declared that he would not be swayed by "partisan interests, public clamor or fear of criticism."

The case played out not far from the suburb of Ferguson, which was the scene of the fatal shooting of Michael Brown, the unarmed black 18-year-old who was killed by a white police officer in 2014. That officer was never charged and eventually resigned.

Stockley, who was charged with first-degree murder, insisted he saw Anthony Lamar Smith holding a gun and felt he was in imminent danger. Prosecutors said the officer planted a gun in Smith's car after the shooting. The officer asked the case to be decided by a judge instead of a jury.

"This court, in conscience, cannot say that the State has proven every element of murder beyond a reasonable doubt or that the State has proven beyond a reasonable doubt that the defendant did not act in self-defense," St. Louis Circuit Judge Timothy Wilson wrote in the decision .

In a written statement, St. Louis Circuit Attorney Kim Gardner acknowledged the difficulty of winning police shooting cases but said prosecutors believe they "offered sufficient evidence that proved beyond a reasonable doubt" that Stockley intended to kill Smith.

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Assistant Circuit Attorney Robert Steele emphasized during the trial that police dashcam video of the chase captured Stockley saying he was "going to kill this (expletive), don't you know it."

Less than a minute later, the officer shot Smith five times. Stockley's lawyer dismissed the comment as "human emotions" uttered during a dangerous police pursuit. The judge wrote that the statement "can be ambiguous depending on the context."

Prosecutors objected to the officer's request for a bench trial. The Constitution guarantees the right of criminal suspects to have their cases heard "by an impartial jury." But defendants can also opt to have the verdict rendered by a judge.

Stockley, 36, could have been sentenced to up to life in prison without parole. He left the St. Louis police force in 2013 and moved to Houston.

The case was among several in recent years in which a white officer killed a black suspect. Officers were acquitted in recent police shooting trials in Minnesota, Oklahoma and Wisconsin. A case in Ohio twice ended with hung juries, and prosecutors have decided not to seek a third trial.

Fears of unrest prompted several downtown businesses and some schools to close early.

Video from St. Louis television stations showed a crowd that swelled from a handful to several hundred in the hours after the verdict and marched through city streets. The group included black and white protesters and some people carrying guns, which is allowed under state law.

Efforts at civil disobedience were largely unsuccessful. When several demonstrators tried to rush onto Interstate 64, they were blocked on an entrance ramp by police cars and officers on bikes. When they tried to enter the city's convention center, the doors were locked.

At one point, a group of the protesters stood in front of a bus filled with officers in riot gear, blocking it from moving forward. When officers began pushing back the crowd, protesters resisted, and police responded with pepper spray, two women told The Associated Press.

Both women's faces had been doused with milk, which is used to counter the effects of pepper spray.

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By early evening, police were saying a protest at a downtown intersection was no longer peaceful and that they were asking demonstrators to leave the area. Protesters had surrounded a police vehicle in front of the old police building near Tucker Boulevard and Clark Avenue and damaged it with rocks. Police approached and tried to secure the vehicle and some in the crowd threw rocks and pieces of curbing at them. Officers then used pepper spray on the group.

In a tweet, the police department said protesters were ignoring commands and violating the law and were subject to arrest.

Thirteen arrests were made and four officers were injured. St. Louis interim police Chief Lawrence O'Toole said none of the officers injured was hospitalized. One had an injured hand, one was hit by a bike, one was pinned by a bike, and one was struck by a water bottle.

Police spokeswoman Schron Jackson said she didn't know if protesters were injured but she was not aware of any reports of demonstrators being hospitalized.

The Rev. Clinton Stancil, a protest leader, said the acquittal was shocking based on the evidence but not surprising.

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"It's a sad day in St. Louis, and it's a sad day to be an American," Stancil said.

One man was arrested for damaging a police vehicle. There were also scattered reports of protesters attacking journalists, authorities said.

The St. Louis area has a history of unrest in similar cases, including after Brown's death, when protests, some of them violent, erupted.

In Smith's case, the encounter began when Stockley and his partner tried to corner Smith in a fast-food restaurant parking lot after seeing what appeared to be a drug deal. Stockley testified that he saw what he believed was a gun, and his partner yelled "gun!" as Smith backed into the police SUV twice to get away.

Stockley's attorney, Neil Bruntrager, argued that Smith, a 24-year-old parole violator with previous convictions for gun and drug crimes, tried to run over the two officers. Stockley fired seven shots as Smith sped away. A chase ensued.

At the end of the chase, Stockley opened fire only when Smith, still in his car, refused commands to put up his hands and reached along the seat "in the area where the gun was," Bruntrager said. Stockley said he climbed into Smith's car and found a revolver between the center console and passenger seat.

But prosecutors questioned why Stockley dug into a bag in the back seat of the police SUV before returning to Smith's car.

The gun found in Smith's car did not have his DNA on it, but it did have Stockley's.