Officers have deployed tear gas rounds into a crowd of protesters in Oregon.
It came hours after the head of the Department of Homeland Security visited Portland and called demonstrators “violent anarchists”.
Senator Jeff Merkley and others sharply criticised federal authorities following reports federal law enforcement was using unmarked vehicles to arrest and detain protesters.
Authoritarian governments, not democratic republics, send unmarked authorities after protesters. These Trump/Barr tactics designed to eliminate any accountability are absolutely unacceptable in America, and must end. https://t.co/PE4YfZ9Vqd— Senator Jeff Merkley (@SenJeffMerkley) July 16, 2020
Oregon Public Broadcasting reported federal officers were detaining people who weren’t near federal property.
“Authoritarian governments, not democratic republics, send unmarked authorities after protesters,” Mr Merkley said in a Tweet.
Video showed many protesters leaving the area near the federal courthouse late on Thursday as smoke filled the air.
Protests have taken place for nearly two months in the city since the police killing of George Floyd in Minneapolis on May 25.
During a visit to Portland earlier on Thursday, Homeland Security acting secretary Chad Wolf said state and city authorities are to blame for not putting an end to the protests, angering local officials.
Mayor Ted Wheeler and other local officials have said they did not ask for help from federal law enforcement and have asked them to leave.
A few hundred people had gathered near the federal courthouse on Thursday night, news outlets reported.
Police told protesters to leave after announcing they heard some chanting about burning down the building, according to The Oregonian.
A short time later, federal officers fired rounds and deployed tear gas to break up the crowd.
Some protesters remained in the area early on Friday and were detained but it was unclear whether any arrests were made, the newspaper reported.
Homeland Security acting deputy secretary Ken Cuccinelli said on Fox & Friends on Friday morning that the US Government has a responsibility to protect buildings such as the courthouse.
“What we’ve seen around the country is where responsible policing is advanced, violence recedes,” Mr Cuccinelli said.
“And Portland hasn’t gotten that memo. Nor have a lot of other cities.
“And the president is determined to do what we can, within our jurisdiction, to help restore peace to these beleaguered cities.”
President Donald Trump recently sent the federal officers to the city.
Tensions have escalated in the past two weeks, particularly after an officer with the US Marshals Service fired at a protester’s head on July 11, critically injuring him.