One person has been shot and for people have been stabbed in protests which saw Donald Trump supporters and counter-demonstrators clash in violent scenes.
Washington State Police said a shooting took place on Capitol Campus in Olympia on Saturday afternoon.
Officers confirmed the suspect – believed to be a member of the far-right group Proud Boys – had been detained.
The shooting came as Conservative groups claiming the US election was stolen from Trump staged protests across the country on Saturday.
Gunfire was heard in Olympia as footage on social media captured the chaotic scenes.
It is not clear how seriously injured the victim is.
Police had declared a riot at around 1.30pm on Saturday after the rival groups started to become violent against each other.
Washington State Patrol said: “Fluid situation with two groups of size, both including heavily armed individuals.
“Sporadic confrontations and movement since late morning.”
Dozens of men wearing riot gear were seen in the area of the shooting.
By 6.40pm, the Olympia Police Department tweeted: “Groups have dispersed.”
At least four people have been left with serious injuries after a stabbing in DC, local journalist Matt Blitz reported on Twitter.
Two police officers and two civilians were stabbed, according to reports.
They were taken to hospital in critical condition, the journalist added.
Violence also erupted between members of the far-right group Proud Boys and Black Lives Matter protesters in Washington DC.
Video posted on Twitter shows a street fight involving people wearing helmets and gas masks, and carrying homemade plastic riot shields.
Conservative groups alleging without evidence that president-elect Joe Biden stole the US election gathered for protests across the country on Saturday.
Few wore masks, despite soaring Covid-19 deaths and cases, defying a mayoral directive for them to be worn outside.
Protests were also held in other communities around the
country, including Atlanta, Georgia, another state where Trump’s campaign has sought to overturn Joe Biden’s election victory, and Mobile, Alabama, according to local news coverage.
The protests happened a day after the US Supreme Court rejected an unprecedented bid backed by Donald Trump to overturn this year’s election results.
A lawsuit filed by Texas and backed by the current American president, who failed to win a second term, was refused on Friday.
It was seeking to invalidate results in Georgia, Michigan, Pennsylvania and Wisconsin – four states President-elect Joe Biden won.
However, the justices said Texas did not have legal standing to bring the case against the four states.