Violent clashes have occurred every night in the city since federal agents arrived.
The White House is planning to send more federal troops to Portland in a bid to quell ongoing protests, reports claim.
Donald Trump’s administration wants to deploy an additional 50 US Customs and Border Protection personnel to the Oregon city, adding to the 100 sent last week, according to the Washington Post.
Federal troops arrived in Portland earlier this month without the consent of local police and politicians, sparking outrage and accusations they were breaching the constitution.
The Black Lives Matter demonstrations had dwindled to around 100 hardcore activists, but numbers shot up into the thousands after Mr Trump’s decision to deploy federal agents.
During the past two weeks of violent clashes authorities have used tear gas and rubber bullets, but their efforts to protect the central courthouse from protesters have failed.
Attorney General William Barr has defended the use of federal troops, saying attacks on the courthouse are “an assault on the government of the United States”.
He will tell Congress in an address on Tuesday: “The most basic responsibility of government is to ensure the rule of law, so that people can live their lives safely and without fear.
“The Justice Department will continue working to meet that solemn responsibility.”
Late on Monday, Mr Trump described demonstrators in Portland as “anarchists” and “agitators”.
He tweeted that any attacks on federal buildings “will be prosecuted under our recently-enacted Statue and Monuments Act”.
The president added: “MINIMUM TEN YEARS IN PRISON. Don’t do it!”
On Sunday a video emerged of a police officer kneeling on a young woman’s neck as they arrested her during the protests.
She was heard shouting: “I can’t breathe” – the last words spoken by Mr Floyd before he died.