"Failure to leave now will subject you to arrest and/or the use of crowd control munitions including but not limited to tear gas and impact weapons", the police said on Twitter https://bit.ly/34BbyTv, adding that personnel were responding to the fire near the city's North Lombard Street and North Campbell Avenue area.
The gathering comes a night after Portland police used tear gas on protesters and made 23 arrests as they declared a demonstration to be a "riot" late on Sunday, saying a group of more than 200 demonstrators lit fires and attacked officers with lasers, rocks and bottles.
That protest took place in the city's North Precinct.
Demonstrations against racism and police brutality have swept the United States since the death in May of George Floyd, a 46-year-old Black man who died after a Minneapolis police officer knelt on his neck for nearly nine minutes.
President Donald Trump's administration in July deployed federal forces to Portland to crack down on the protests.
On Friday, he denounced the demonstrations as "crazy" and said cities run by Democrats had descended into chaos. Portland Mayor Ted Wheeler is a Democrat.
Read More: usnews.com