Several people were seen being detained, but it was not immediately clear how many may have been arrested.
Law enforcement spent hours working to disperse the crowd. At times, protesters scattered amid flash bangs.
Officers deployed more tear gas and less-lethal munitions, and formed a line to push back demonstrators. Many dispersed, but several protesters remained well after 2am.
It's become an all-too-familiar scene in Portland.
Authorities declared a riot early Sunday morning. Protesters had breached a fence surrounding the city’s federal courthouse building where US agents have been stationed.
Protesters remained in the streets past 2:30am, forming lines across intersections and holding makeshift shields, as police patrolled and closed blocks abutting the area.
Portland police say they arrested six people.
Sunday evening, Portland police had responded to a shooting at Lownsdale Square Park, near where the overnight protests have been taking place.
Two people were taken into custody and later released, police said Monday morning.
The person who was shot went to the hospital in a private vehicle and was treated for a non-life-threatening wound.