Hundreds of people protesting the acquittal of a white former St Louis 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 and other potential protest sites.

A racially diverse crowd of protesters - some carrying weapons and others toting children and waving posters - took to the streets within hours of Friday's decision.

More than 20 arrests were made and some protesters were pepper-sprayed during confrontations with authorities. St Louis police reported that 10 officers had suffered injuries, including a broken jaw and dislocated shoulder, and some journalists reported being threatened by demonstrators.

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Activists were to meet today to plan further demonstrations.

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

Stockley, 36, requested his case be decided by a judge alone.

"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 ruled.

Assistant Circuit Attorney Robert Steele emphasised 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".

Stockley left 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. Stockley said he found a revolver between the centre console and passenger seat of Smith's car.

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.

- AP