ST LOUIS, USA (AP) — Hundreds of people protesting the acquittal of a white former St Louis police officer in the fatal shooting of black man marched for hours on Friday in mostly peaceful demonstrations, until a broken window at the mayor’s home and escalating tensions led riot police to lob tear gas to disperse the crowds.

For weeks, activists had been threatening civil disobedience if Jason Stockley were not convicted of murder for killing Anthony Lamar Smith, prompting authorities to take precautions. With the large protests that followed the 2014 killing of Michael Brown in nearby Ferguson still fresh in everyone’s minds, barricades were erected around police headquarters and the courthouse, among other sites, in anticipation of the verdict.

Within hours of St Louis Circuit Judge Timothy Wilson acquitting Stockley of first-degree murder, a racially diverse crowd of protesters took to the streets — some legally carrying weapons and others toting children and waving posters.

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 threatened by protesters.

Activists said they would meet again to plan further demonstrations. The band U2 cancelled its night concert in St Louis set for last night because the police department said it wouldn’t be able to provide its standard protection for the event, organizers said.

The 2011 confrontation began when Stockley and his partner tried to arrest Smith for a suspected drug deal in a fast-food restaurant. Smith sped off, leading to a chase that ended when he crashed.

At the trial, Stockley testified that he saw the 24-year-old Smith holding a silver revolver as he sped away at the start of the chase. He said when he shot Smith, he felt he was in imminent danger.

Prosecutors said Stockley planted a gun in Smith’s car after the shooting

Dashcam video from Stockley’s police car captured him saying he was “going to kill this (expletive), don’t you know it.” Less than a minute later, he shot Smith five times.

Stockley’s lawyer dismissed the comment as “human emotions” uttered during a dangerous pursuit.

In his decision, Wilson wrote that the statement “can be ambiguous depending on the context.”

“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,” the judge wrote.

In an interview with the St Louis Post-Dispatch after the verdict, Stockley, 36, 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,” said Stockley, who left St Louis’ police force in 2013 and moved to Houston.

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.

Early confrontations erupted when protesters blocked a bus full of officers in riot gear and later surrounded a police vehicle that was damaged with rocks, prompting police to deploy pepper spray.

Tensions escalated after protesters broke a front window and splattered red paint on 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.