US agents repeatedly fired what appeared to be tear gas, flash bangs and pepper balls early on Monday to clear a mass of protesters outside the Mark O. Hatfield United States Courthouse in Portland.

Some protesters had climbed over the fence surrounding the courthouse, while others shot fireworks, banged on the fence and projected lights on the building.

The federal courthouse has been a place of contention as the city has seen nightly protests for two months since the death of George Floyd in Minneapolis in May.

President Donald Trump said he sent federal agents to Portland to halt the unrest but state and local officials said they are making the situation worse.

The protest late on Sunday started peacefully, with organisers giving speeches and leading chants with the crowd.

But things intensified as the night wore on. US agents declared an "unlawful assembly," and just after 1am, confronted protesters on the street and worked to clear the area. 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 2 am

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.

On 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.

Also on Sunday, police said someone pointed out a bag in the same park, where officers found loaded rifle magazines and Molotov cocktails. The shooting and the items in the bag were not related, police said. It's unclear whether either incident was connected to the protest.

Portland police said the department's officers were not involved in dispersing the crowds during the protest early on Monday.

Australian Associated Press