Seattle Police say around 50 protesters marched along 12th Avenue from a juvenile detention center near Seattle University. The situation escalated shortly before 9 p.m., when police say protesters threw rocks and bottles at them.
At least one squad car had a door dented and had paint tossed on it.
With handcuffs around his wrists and officers on both sides, protester Ian Finkenbinder said this was about sending a message for children.
"We're protesting the imprisonment and enslavement of children in the prison/industrial complex in the form of juvenile detention," Finkenbinder said.
Other people said they were with the group Anonymous and said police were egging them on and being antagonistic.
An officer said some of the protestors fled onto the Seattle University campus. The protesters shed clothes and gear along the way and an officer said they flushed out some of the fleeing people.
SPD did not have a final tally on arrests, but it appears there was no other major damage.