Over the course of the evening, "anarchists" in the crowd threw objects at officers, vandalized local businesses and damaged cars, Portland Police Sgt. Pete Simpson said.
Police publicly declared a "riot" due to "extensive criminal and dangerous behavior" and called the protest "unlawful," according to posts on the department's Twitter page.
The crowd was dispersed using "less lethal munitions" and at least 26 protesters were arrested, police said.
Protests and marches continued in other U.S. cities including Los Angeles, Philadelphia, Denver, Minneapolis, Baltimore, Dallas, and Oakland, CA.
Some of the roughly 1,000 protesters in Oakland broke store windows, left graffiti on buildings and threw M-80 firecrackers, Molotov cocktails and bottles at police officers, authorities said.
Eleven people were arrested in Oakland, including someone who had seven Molotov cocktails in his possession, police said. Some of those arrested are accused of assaulting police officers.
In Los Angeles, about 185 protesters were arrested Thursday night into Friday morning, police Officer Norma Eisenman said -- including some on suspicion of blocking roads, and minors accused of violating curfew.