The agency's statement is in response to US media reports and images on social media showing federal agents in military gear taking several protesters away in unmarked vehicles amid protests near a federal court house on Thursday night.
"CBP agents had information indicating the person in the video was suspected of assaults against federal agents or destruction of federal property," the release said on Friday. "Once CBP agents approached the suspect, a large and violent mob moved towards their location. For everyone’s safety, CBP agents quickly moved the suspect to a safer location for further questioning."
The images drew criticism from Democratic lawmakers and rights groups claiming it was unconstitutional and wrong.
The CBP said "violent anarchists" have been behind the demonstrations in Portland the past several weeks, claiming that their intentions are to destroy federal property and to injure federal officers.
US media reported that the federal response has been limited to federal property so far.
Portland, a stronghold for anarchist groups and far-left activists has been subject to protests and riots since the 25 May death of African American man George Floyd while in police custody, with businesses suffering millions of dollars in losses from destroyed property and shoppers who stayed away, according to media reports.