The police said it had learnt that people near the PPA building in the city's North Lombard Street "may be attempting to force entry into the building" after a march of about 200 people stopped at the building, causing traffic to be blocked, Reuters reported.
"Failure to follow this order may subject you to the use of crowd control agents, including, but not limited to tear gas and/or impact weapons," the police said in a tweet.
"Anyone participating in criminal behavior, including burglary and/or vandalism, are subject to arrest and/or citation."
In Portland, some demonstrators have previously attacked a federal courthouse and others have gathered to speak out against racism and police brutality following the May 25 death of George Floyd.
Floyd, a 46-year-old African-American man, died after a white police officer knelt on his neck for nearly nine minutes.
US Attorney General William Barr came under fire from Democratic lawmakers earlier this month for sending federal officers to disperse protesters in Portland.