Portland has been the scene of unrest for months, with civil rights, anarchist and anti-fascist protesters scuffling with police and occasionally with right-wing militias and Trump supporters.
The protests began in the summer as part of nationwide demonstrations against racism and police brutality following the death of George Floyd, a Black man who died in Minneapolis in May after police pinned his neck to the ground for nearly nine minutes.
Eight people were arrested in Portland on suspicion of offences including rioting and reckless burning, while two people were arrested in Seattle, one for assault and another for property damage, police said.
Before descending on the Democratic Party office, some protesters from a group of about 150 scuffled with police on bicycles, police said in a statement.
In Seattle, police said multiple sites had been vandalised and posted pictures of the damage, including what appeared to be of an Amazon Go store.