Police arrested 18 protesters early Saturday as thousands gathered in St. Paul, Minn., after a jury acquitted a police officer who shot and killed Philando Castile.

Minnesota State Police began arresting some people after issuing three warnings for them to get off Interstate 94, according to the Associated Press.

Photos: Protests erupt after officer acquitted in killing of Philando Castile

Castile, a black motorist, was killed during a traffic stop last July by officer Jeronimo Yanez in Falcon Heights, a St. Paul suburb. The shooting received national attention when Castile’s girlfriend live streamed the moments after on Facebook Live. No video existed of exactly what happened inside the car.

A jury acquitted Yanez Friday, clearing him of a manslaughter charge as well as two lesser charges.

Thousands came together on the steps of the state Capitol in St. Paul late Friday, holding signs and banners and chanting in response to the not guilty verdict. After the hour-long rally, protesters took to the streets. Saint Paul Police escorted the crowd, which they estimated to be about 2,000 people.

The protest was peaceful, but a smaller group splintered off and walked down an entrance ramp to block Interstate 94, quickly snarling traffic and leading to the freeway’s shutdown in both directions, AP reported.

There were also protests last July after the shooting.

Castile’s mother, Valerie, who addressed the crowd Friday after leaving the court, blasted the decision.

“People have died for us to have these rights and now we're devolving,” she said. “We're going back down to 1969. Damn. What is it going to take? I'm mad as hell right now, yes I am.”

Castile said her son would never jeopardize someone's life, especially with his girlfriend and her child in the car at the time.

"I will continue to say murder because where in this planet do you tell the truth and you be honest and you still be murdered by the police of Minnesota?" Valerie Castile asked.

Others, including some members of Congress, also criticized the court decision.

“This verdict tells African Americans across the country that they can be killed by police officers with impunity, even when they are following the law, and that it is reasonable for any person interacting with an African American in any way to fear for his or her life,’’ Rep. Cedric Richmond, D-La., chairman of the Congressional Black Caucus, said in a statement. “We need to work at all levels of government to create a system of justice that upholds the rights of every American, whether they are black or white.’’

Castile’s death was one of several killings of black men by police officers that garnered national attention and sparked protests. Others include, Alton Sterling in Baton Rouge, La., Eric Garner, who was held in a chokehold by police, and died in New York and Michael Brown, an unarmed teenager, who was shot by an officer in Ferguson, Mo.