The Seattle Police Department (SPD) tweeted about the incident and provided pictures of the scene along with additional details.
"Group of approximately a dozen people setting fire and causing damage to a portable trailer and construction site at 12/Jefferson. Possible explosions heard on site. Large group in the area. Working to secure access for," the Twitter thread began.
Law enforcement said protesters also broke windows and damaged vehicles near a King County Court facility. There were also reports of businesses being destroyed and vandals spraypainting the East Precinct while attempting to disable security cameras near the perimeter.
"Due to the ongoing damage and public safety risks associated with this incident, SPD is declaring it a riot," another tweet read.
SPD later released details about the injured officers, saying that one was hospitalized as the result of one of the explosions, while two other officers received medical treatment and were able to return to duty.
The department said 16 arrests were made for "assault on officers, obstruction and failure to disperse."
The incident comes as a new law was set to take effect in Seattle on Sunday that “bans Seattle Police officers the use of less-lethal tools, including pepper spray that is commonly used to disperse crowds that have turned violent,” police Chief Carmen Best said in a statement. It would also have prohibited the use of anti-riot gear.
“Simply put," Best added, "the legislation gives officers NO ability to safely intercede to preserve property in the midst of a large, violent crowd."
U.S. District Judge James Robart on Friday granted a request by the federal government to block the measure, the Seattle Times reported.
Fox News' Caitlin McFall and Dom Calicchio contributed to this report