At least two demonstrators were arrested amid the violence throughout which windows at Portland State University's public safety office were shattered.
Demonstrators also launched a flammable liquid inside the Starbucks, whose walls were graffitied with 'BLM' and 'ACAB', which stands for 'all cops are b******s'.
The Multnomah County Sheriff's Office announced they ended the 'possible arson' and declared an unlawful assembly, directing the group to leave.
Protesters had gathered in a park at 9 pm and walked together chanting the name of Jason Washington, a black man shot and killed by PSU campus police in 2018.
WATCH: BIDEN CLAIMS INFLATION WAS 9% WHEN HE GOT IN OFFICE
Police said that Kaiave James Douvia, 22, was arrested for criminal mischief, burglary and disorderly conduct, and Connor Austin, 25, was detained for meddling with a peace officer.
They also seized a tire iron, a dumbbell, body armor, gas masks and an umbrella.
WATCH VIVEK: "WHAT IS THE CRIME THAT DONALD TRUMP COMMITTED?"
Unrest has seized Oregon city for more than 100 days after the death of George Floyd, and it has grown a symbol of the US's stark divisions.
It comes as the FBI warned of the possibility for more armed conflicts in Portland associated with the US election, with the National Guard being readied for riots.
NIGHT OPERATION UNCOVERS THE TRAGIC FATE OF THREE MUSICAL FESTIVAL HOSTAGES IN GAZA TUNNEL
The fiercely polarized vote, which could see President Donald Trump reelected, or possibly defeated by his Democratic opponent Joe Biden, has pushed fears of more dangerous street violence.
Downtown businesses were boarding up windows once again as riots are planned for either a Trump or a Biden victory, or even a state of limbo, with delays in the vote-counting expected nationwide due to a flood in mail-in voting throughout the pandemic.
CAPITOL RECOGNITION: A TRUE AMERICAN EVANGELIST EARNS BIG HONOR...
'The thing that is the most concerning to me is the potential for armed clashes between opposing groups,' FBI Portland Special Agent Renn Cannon told AFP.
BIDEN’S HONORARY DEGREE: A SYMBOL OF DIVISION AT MOREHOUSE COLLEGE?
'That could escalate into a dangerous situation where, if tempers are heated, you could end up with an unfortunate or tragic act of violence,' he continued, pointing to a fatal shooting of a far-right fan in the city in August.
The 250-strong Portland office has dedicated additional resources to election crimes, even voter suppression, fraud, and foreign cyber threats, said Cannon.
WILL BIDEN’S DEBATE DEMANDS TURN THE TIDE OR DROWN HIS CHANCES?
Juanita Broaddrick, the Republican former nurse who alleged that U.S. President Bill Clinton raped her in April 1978 took to twitter about the dangerous situation writing her concerns: “When the Democrats sent their BLM and Antifa thugs into our communities to cause violence. death and destruction.......the dynamics changed. Now..... it’s personal. We will go through hell or high water to vote for Donald Trump.”
Discover alternative ideas that will make you think
Engage in mind bending debate
Earn points, rise in rank, have fun
