Police in Portland, Oregon, declared an unlawful assembly Saturday after antifa protesters clashed with far-right demonstrators, including the Proud Boys, in the downtown area, resulting in multiple injuries.

Authorities said they received reports that some demonstrators during the noontime action threw "milkshakes" that were actually cups mixed with quick-drying concrete, a powder-and-water mixture that can set in as little as 10 minutes and cause physical harm.

"Police have received information that some of the milkshakes thrown today during the demonstration contained quick-drying cement," the Portland Police Department tweeted. "We are encouraging anyone hit with a substance today to report it to police."

As of Monday morning, there was no physical evidence or additional reports of the use of the so-called "concrete milkshakes." A request for comment sent to the Portland Police Department was not immediately returned.

A woman was arrested for allegedly throwing a substance, police said in a statement.

"During today's events, there were multiple assaults reported, as well as projectiles thrown at demonstrators and officers," police said. "There were also reports of pepper spray and bear spray being used by people in the crowd. Officers deployed pepper spray during the incident."

Authorities declared an unlawful assembly shortly after 3 p.m. and said the demonstrations had descended into a "civil disturbance." Officers in riot gear cleared downtown and Pioneer Courthouse Square, warning that stragglers would be arrested.

Other arrests included a man for alleged harassment and a woman for suspected disorderly conduct and harassment.

Among eight people injured were two officers who were pepper sprayed, one who was punched in an arm, and one who was hit with a "projectile." Additionally, three civilians were "assaulted with weapons," according to the statement, which did not account for the eighth injury.

"We are actively investigating these incidents to hold those responsible accountable," Assistant Chief Chris Davis said.

NBC News affiliate KGW in Portland reported that hundreds of protesters had gathered Saturday, including anti-fascists on the left and members of the Proud Boys and the "HimToo" movement on the right, according to KGW and the Portland Oregonian newspaper.

The area cleared out by late afternoon.