The Portland Police Bureau said that lead balls, smoke bombs, paint, glass bottles and full cans of Pepsi were "launched" at police. Officers were forced to put on protective helmets and other protective equipment due to the items that were being thrown at them, police said. At one point there were attempts to set a business on fire, according to police.
The bureau on Tuesday shared mugshots for 22 of the 25 suspects who were arrested in "riots" that broke out at the march. Police said the suspects, most of whom were arrested for disorderly conduct, ranged from ages 17 to 44.
A window was also broken at the Edith Green-Wendell Wyatt Federal Building, police said.
Police initially encouraged peaceful protesters to separate themselves from "anarchists committing acts of violence/property damage." Authorities also advised that families with children at the march to "immediately leave" due to the mounting violence.
Just before 4:30 p.m., police said the permit for the march had been canceled and informed everyone in the vicinity to "leave the streets" due to the "unsafe conditions." The event was declared a riot by 5 p.m.
After announcing that the permit had been canceled, Portland police asked law-abiding participants to use the sidewalks and leave the area, adding that anyone remaining in the street would be subject to arrest.
Witnesses said it was initially a peaceful and family-friendly event.
"It's frightening, and I think, moreover, it's just disgusting and depraved behavior," Keely Bottcher, who witnesses some of what chaos, told ABC affiliate KATU on Tuesday. "I just wonder what happened to the idea of democracy and being American. And (it) seems like everything is turning very political. You can't like democracy only if it goes your way."
Police said "impact munitions" and "chemical munitions" were deployed on the crowds due to the "numerous projectiles being thrown at police."
Protesters then began to start fires and throw "incendiary devices" at officers, including fireworks, smoke bombs and Molotov cocktails, police said. One police car was destroyed by protesters. Multiple businesses, including a Target and a local jewelry store, were damaged in the riots.
Cal Brockman, the owner of Goldmark Jewelers, said a protester threw a rock through his business' window.
"Still, it's difficult to clean up and to fix things and get them put back, because there's a fair amount of things that are smaller-scale damage -- things that got bent, things that got scratched," Brockman told KATU on Tuesday. "And so to put it back to where it was, it's going to take time."
May Day, which falls on the first day of May, is also known as International Workers' Day, which has long been an occasion for political action by labor unions and other progressive organizations.
Demonstrations were planned on Monday in major cities including New York Seattle, San Francisco and Los Angeles.
ABC News' Michael Hayden and Marilyn Heck contributed to this report.