Officers used stun grenades and pepper spray, as protesters set a fire and broke windows. The march was in support of ongoing protests in Portland.
Forty-five people were arrested while 21 officers were injured.
In Austin, Texas one man was killed during a Black Lives Matter march.
Police said initial reports suggested the victim might have been carrying a rifle and approached a vehicle, from where a person shot and killed him. The suspect has been arrested and is co-operating with officers.
The demonstrations have been given renewed energy by violent clashes in Portland between protesters and federal agents deployed by President Donald Trump despite opposition from local and state leaders.
In Seattle, thousands of protesters had initially gathered peacefully, carrying signs such as “Feds go home” and “We are living in a police state”, and shouting chants of “No justice, no peace”.
A group then set fire to the construction site for a youth detention facility before smashing windows of a courthouse and nearby businesses, police said. Authorities said rocks, bottles, fireworks and mortars were thrown at officers, and one of them was taken to hospital with a leg injury.
Police declared the demonstrations a riot and said they were investigating whether an explosive device was used against a police station. No injuries were reported.
Like Portland, Seattle has seen extended protests against racism and police brutality since the death of George Floyd in police custody in May. But after a police-free protest zone in the city was dismantled earlier this month following a series of shootings, demonstrations had waned.
What happened elsewhere in the US?
A car drove through a crowd in Aurora, Colorado but there were no reports of injuries. At the same march, a person was injured after a protester “decided to fire off a weapon”, police said. The person is reportedly in a stable condition in hospital.
Demonstrators in the city also remembered Elijah McClain, a 23-year-old black man who died last August after being stopped by police.
In Louisville, Kentucky hundreds of members of a black militia demanded justice for Breonna Taylor, a 26-year-old black woman who was fatally shot when officers entered her flat in March.
Carrying semi-automatic rifles and shotguns, the group walked in formation to a fenced off intersection where they were separated by police from a smaller crowd of armed counter-protesters. There were no reports of incidents.
Earlier, three members of the black militia were taken to a hospital with non-life-threatening injuries after a weapon was accidentally discharged. Police said the incident was being investigated.
What is happening in Portland?
Mr Trump’s decision to send federal law enforcement agents to protect government buildings in Portland, Oregon, has been deeply controversial. Clashes have escalated recently.
Federal officers in unmarked vehicles appeared to forcefully seize protesters from the streets and detain them without justification. They have also fired tear gas and less-lethal munitions into crowds of demonstrators.
President Donald Trump says he is trying to restore order but his approach has drawn widespread criticism and legal challenges.
The Democratic governor of Oregon, Kate Brown, has demanded their withdrawal, and local officials say this is an election-year ploy by the president to try and paint his opponents as weak on law and order.