Officials declared a demonstration in Portland, Oregon, a "riot" on Thursday evening, activating the National Guard, after protesters smashed shop windows.
The city became the first in the US to be patrolled by the Guard during post-election unrest, which has hit cities including Detroit, Los Angeles and Phoenix.
Hundreds of Black Lives Matter protesters marched towards the waterfront in Portland's city centre, where they met another crowd, black-led J.U.I.C.E PDX - which stands for "Justice, Unity, Integrity, Community, Equality", and combined forces.
The event by the Columbia River remained peaceful and called for racial justice, with the majority of attendees dispersed by 8.30pm.
However a splinter group of protesters, the majority of them white, broke off from the Black Lives Matter march and headed towards downtown, where they vandalised carpark pay machines, smashed business windows and graffitied on walls.
At about 7pm, the Multnomah County Sheriff's Office declared the protest a riot, while Governor Kate Brown activated the use of the state National Guard to assist the Oregon State Police and Sheriff's Department in curbing the unrest.
"It's important to trust the process and the system that has ensured free and fair elections in this country through the decades, even in times of great crisis," Ms Brown said in a statement. "
We are all in this together — so let's work together to keep our fellow Oregonians safe."
After one man set fire to the American flag, police moved in to make arrests, warning protesters the gathering was unlawful and CS gas may be used if they did not disperse.
Officers arrested at least 11 people, including one person who was accused of throwing a Molotov cocktail at police, and The National witnessed an officer pushing one female protester to the ground, though she did not need to receive medical treatment.
Sheriffs also said they seized a rifle, ammunition and fireworks from those arrested.
Just two hours later, another crowd gathered outside the Justice Centre, rallied by a man who urged the group to walk through downtown again.
"Did we just come here to be quiet?" he asked the crowd. "Did we just come here to sit in a park and chat?"
The protesters, most of them dressed head to toe in black and wearing black facial coverings, made their way back through downtown again.
Despite Mayor Ted Wheeler, who is also police commissioner, banning the use of tear gas after concerns over an aggressive response to the unrest, law enforcement warned the protesters they would be gassed if they did not comply.
Just after 11pm, Humvees arrived carrying a number of National Guard officers, who stood alongside police brandishing wooden batons.
"Oregon National Guard members are civilian community members helping to protect us," said a statement issued by Ms Brown's joint command. "We don't take this decision lightly."
Thursday evening's was the third consecutive protest to take place in the city, which has seen almost five months of unrest prompted by the death of George Floyd at the hands of police.