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The arrests came hours after St. Anthony Police Officer Jeronimo Yanez was found not guilty on Friday of second-degree manslaughter in 32-year-old Castile's July 2016 shooting death in the St. Paul suburb of Falcon Heights.
The shooting drew national attention after the victim's girlfriend live-streamed the bloody aftermath on social media, and it led to protests that have fueled debate across the country over police use of force in encounters with minorities.
Protesters held a peaceful demonstration on Friday at the Minnesota State Capitol in St. Paul and then about 1,500 converged on Interstate 94 in the city, blocking the freeway, a Minnesota State Patrol spokeswoman said in an email statement.
Authorities repeatedly asked protesters to leave before making 18 arrests shortly after midnight on Saturday, the statement said. Unlawful assembly and other charges are pending against them, it said.
Among those listed as arrested for alleged unlawful assembly were at least two journalists.
Yanez, the son of a Mexican immigrant, testified during the trial in Ramsey County District Court that he feared for his life after Castile began reaching for a firearm that Castile had disclosed he had in his possession. Yanez shot Castile five times.
U.S. Senator Al Franken of Minnesota on Saturday said Castile did not deserve to die.
"Whatever one's opinion of the outcome of this case, we must come together and take concrete action to reckon with and dismantle the systemic racial inequalities that lead to far too many of these deaths," Franken wrote on Facebook.
The video footage of the aftermath of the shooting taken by Castile's girlfriend had shaped many public perceptions of the fatal shooting before the trial.
The video begins with the girlfriend, Diamond Reynolds, in the passenger seat as Castile, covered in blood, sits in the driver's seat and a patrolman points his gun into the vehicle.
"He was trying to get out his ID and his wallet out of his pocket," Reynolds says. "He let the officer know that he had a firearm and that he was reaching for his wallet, and the officer just shot him in his arm."