In video circulating online, a woman can be seen walking towards police and spitting at them once before she is wrestled to the floor by a group of officers wearing riot gear.
“Police brutality cannot be tolerated on any level," Riley Renn, the woman who filmed the video, told The Independent.
“Many of the people who were counter-protesting were also there to speak out against [the] lack of accountability when it comes to the United States and our massive police presence.”
It is not clear whether the woman involved was part of an official protest or just a passer-by in the area.
Portland Police have been contacted for comment on the incident shown in the video.
At least 13 people were arrested in the city on Saturday as officers seized metal poles, bear spray and other weapons during the right-wing rally.
On Saturday, Mr Trump showed support for one of the demands of right-wing demonstrators by saying “major consideration” was being given to naming the anti-fascist group Antifa an “organisation of terror”.
Leaders of right-wing groups have vowed to return to the city so long as Antifa groups remain active in the area.
However, Portland’s mayor Ted Wheeler said Joe Biggs, the organiser of the event, was not welcome in the city.
"We do not want him here in my city. Period,” Mr Wheeler said, adding that he believed the demonstrations were linked to “a rising white nationalist movement” in the US.
"Portland, being a very progressive community, is always going to be at or near ground zero of this battle,” he said.
More than two dozen local, state and federal law enforcement agencies, including the FBI, were in the city for the event, which was attended by members of the far-right Proud Boys group and Three Percenters militia movement.
Lieutenant Tina Jones, a police spokesperson, said there were about 1,200 people on the streets at the peak of the protests.
The Proud Boys, who organised the event on Saturday, have been designated a hate group by the Southern Poverty Law Centre.
The rally was inspired by a viral video of conservative blogger Andy Ngo being attacked by anti-fascist protesters in June.
Agencies contributed to this report