Hundreds of people protesting the acquittal of a white former St. Louis, Mo., police officer in the fatal shooting of black man following a high-speed chase marched for hours in mostly peaceful demonstrations, until a broken window at the mayor's home and escalating tensions led riot-gear-clad officers to lob tear gas to disperse the crowds.

Activists had for weeks threatened civil disobedience if Jason Stockley were not convicted in the death of Anthony Lamar Smith, stirring fears of civil unrest and the erecting of barricades around police headquarters, the courthouse where the trial was held and other potential protest sites.

A racially diverse crowd of protesters — some carrying weapons, as allowed by state law, and others toting children and waving posters — took to the streets within hours of Friday's decision.

More than 20 arrests were made by early evening, and some protesters were pepper-sprayed during confrontations with authorities. St. Louis police reported that 10 officers had suffered injuries by the end of the night, including a broken jaw and dislocated shoulder, and some journalists reported being the target of threats from demonstrators.

Activists said they would meet again Saturday to plan further demonstrations.

Stockley, who was charged with first-degree murder in the 2011 shooting, had insisted he saw the 24-year-old 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.

People march downtown after the acquittal in the murder trial of Jason Stockley, a former St. Louis police officer charged with the 2011 shooting of Anthony Lamar Smith, who was black, in St. Louis, Mo., Friday. (Whitney Curtis/Reuters)

In an interview with the St. Louis Post-Dispatch, Stockley said he understands how video of the shooting looks bad, but that he did nothing wrong.

"I can feel for and I understand what the family is going through, and I know everyone wants someone to blame, but I'm just not the guy," he said.

Stockley, 36, requested his case be decided by a judge instead of a jury, an option open to any defendant.

"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-defence," St. Louis Circuit Judge Timothy Wilson wrote in his decision.

St. Louis Circuit Attorney Kim Gardner acknowledged the difficulty of winning police shooting cases but said prosecutors believe they proved that Stockley intended to kill Smith.

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."

A woman marches in downtown St. Louis after after Stockley was acquitted. (Whitney Curtis/Reuters)

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."

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

1 of several officer acquittals

The case was among several in recent years in which a white officer killed a black suspect, including in the nearby suburb of Ferguson, Miss., where Michael Brown, an unarmed black 18-year-old, was killed by a white police officer in 2014. That officer never was charged and eventually resigned.

Officers also have been acquitted in recent shooting trials in Minnesota, Oklahoma and Wisconsin. Friday's protests began with largely unsuccessful efforts at civil disobedience. Demonstrators were blocked on an entrance ramp before they could rush onto an interstate, and found the city's convention center's doors locked when they tried to enter.

Early confrontations erupted when protesters blocked a bus filled with officers in riot gear, and later surrounded a police vehicle that was damaged with rocks, prompting police to deploy pepper spray. A freelance Associated Press videographer said a protester threw his camera to the ground and damaged it, and he was threatened with a beating if he didn't put another camera away. A KTVI reporter said water bottles were thrown at him after a protester taunted him, drawing a crowd.

A protester is treated after being pepper sprayed by St. Louis police during a standoff with protesters after Stockley's acquittal. (Whitney Curtis/Reuters)

As night fell, hundreds of demonstrators walked through the streets to the city's upscale Central West End section, where they chanted and marched as people looked on from restaurants and hospital windows lining busy Kingshighway.

Tensions escalated after protesters broke a front window and splattered red paint at the home of St. Louis Mayor Lyda Krewson, who had called for calm ahead of the verdict and later said she was appalled by what happened to Smith and "sobered" by the outcome.

Police in bulletproof vests and helmets closed in and demanded protesters get off the lawn and out of the street in front of the house. Officers used tear gas to move the crowd out of the area and the protests were all but dispersed within the next two hours.

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

The encounter with Smith 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 tried to run over the two officers. Stockley fired seven shots as Smith sped away. A chase ensued.

People march downtown after the not guilty verdict was announced. (Whitney Curtis/Reuters)

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.