Authorities said the National Guard was deployed because there was "widespread violence" happening in downtown Portland, especially in the area of Southwest 10th and Harvey.
Police said the man was believed to have thrown a Molotov cocktail at Portland police officers.
Sgt. Kevin Allen, a spokesman for the unified command, said in a video on Twitter just after 8 p.m., that rumors of an unmarked van being used to arrest people weren't true.
"Arrestees are being transported in marked police vehicles. Some law enforcement are using unmarked vans to transport official personnel and may be seen downtown," he said.
Officials said Gov. Kate Brown had authorized the use of guard members to help local authorities in "the interest of public safety."
Protesters started gathering Wednesday evening at Revolution Hall in Southeast Portland for a "Defend Democracy" rally before marching across the Morrison Bridge to Waterfront Park.
The group met up with a second protest group on Naito Parkway.
The other demonstration started in Portland’s North Park Blocks earlier that afternoon. That group started marching east by 5 p.m. to meet up with the "Defend Democracy" group at the waterfront. The Multnomah County Sheriff's Office said members of the second group were seen smashing windows and vandalizing property. A KATU reporter on the ground also observed shattered windows at businesses.
Authorities said objects, including a Molotov cocktail and glass bottles, were thrown at them.
From Chopper 2, a line of police could be seen working to disperse a crowd from a downtown street. Crowds of people were seen fleeing from police.
Police said they seized several items, including fireworks, from people who were arrested.
Demonstrations that took place Tuesday on Election Night remained peaceful.
Brown on Wednesday extended an executive order that tasked Oregon State Police and the Multnomah County Sheriff’s Office with keeping the peace at protests surrounding the November election.
The order was initially set to expire Wednesday evening, but the governor extended the order through Friday.
This is a breaking news story and will be updated.