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May Day protests turned violent Monday in Portland, Oregon, where police canceled protest permits, arrested "numerous" people and used gas and non-lethal weapons against demonstrators whom they described as anarchists.

Other May Day gatherings around the world were relatively peaceful, but police geared up for confrontations in the Pacific Northwest's biggest cities, Portland and Seattle, where previous protests have created havoc.

Rallies began at noon (3 p.m. ET) in Portland. Police reported that members of an anarchist group began throwing rocks, lead balls, full cans of soda, glass bottles and "incendiary devices" late in the afternoon. Authorities canceled parade and protests permits about 4:30 p.m.

"If you do not need to come to Downtown Portland, please stay away," police said as firefighters struggled to reach multiple blazes that authorities said were set by protesters. "May Day is now considered a riot."

Broken windows outside Target store in downtown Portland. #MayDay2017 pic.twitter.com/IzABhAo8Ha — Kyle Iboshi (@KyleIboshi) May 2, 2017

Police would confirm only that at least three people had been arrested, but NBC affiliate KGW reported that many more people could be seen detained lying on the street.

Organizers told KGW that they didn't believe the self-described anarchists intended to spur violence — "they are doing their own thing," according to Lili Elbe, a spokeswoman for the Portland May Day Coalition.

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In Seattle — the scene of routinely tense protests ever since riots protesting the World Trade Organization put the city at the forefront of liberal activism in 1999 — police said they were expecting multiple unpermitted events through the evening.

Police in riot gear monitoring the protests moved in on bicycles to separate groups supporting and opposing President Donald Trump at downtown Seattle's busy pedestrian Westlake Park.

Related: Milwaukee May Day March Targets Sheriff Who Let Inmate Die After 7 Days Without Water

South of Seattle, in Olympia, the state capital, police said Monday night that protesters were targeting officers and journalists with rocks, bottles and pepper spray. Some were using slingshots to propel rocks, police said.

"This is a riot," the police department said. "We are ordering the mob to disperse. If you do not disperse you are subject to arrest."

At least eight people had been detained by 7:45 p.m. (10:45 p.m. ET), police said.

Mob is breaking windows on businesses. Leave the area or be subject to arrest. This mob is trying to harm people. We are engaging — Olympia Police Dept (@OlyPD) May 2, 2017

Elsewhere:

More than a dozen demonstrators were arrested outside JPMorgan Chase on Park Avenue in New York City on Monday morning, NBC New York reported, but rallies at Bryant Park, Union Square Park and Washington Square Park were peaceful.

In Oakland, California, four people were arrested after they chained themselves to the front doors of the Alameda County Administration Building, demanding that the county change how it works with federal immigration officials, NBC Bay Area reported.

Police in riot gear moved in and detained at least one person in Montreal Monday night as an anti-capitalist march deteriorated, with projectiles flying through the air, the CBC reported.

A notice was given to the protesters regarding the vandalism that occured. Everyone must respect laws/regulations.#manifencours #MayDay2017 — Police Montréal (@SPVM) May 1, 2017

Downtown Grand Rapids, Michigan, was largely shut down as a four-hour march made its way slowly through the city, NBC affiliate WOOD reported.

One of the largest protests was planned for Los Angeles, where tens of thousands of people were expected.

"We are being challenged like never before, and we have to step up to the plate and deal with the situation and offer some hope to the community," Juan Jose Gutierrez, coordinator of the Full Rights for Immigrants Coalition, told NBC Los Angeles.

May 1 has been observed as International Workers Day for more than 130 years, but it has become more prominent in the United States since 2006, when major demonstrations were held across the country to support immigrants — a cause that many groups say is especially important during the Trump presidency.

Related: Latino Immigrants, Workers Rally on May Day for 'Day Without Immigrants'

Several hundred people gathered in front of the Statehouse in Boston to urge lawmakers and Gov. Charlie Baker to designate Massachusetts a "sanctuary state." The proposal would restrict state and local law enforcement officers from cooperating with federal immigration efforts.

"We want to be more visible, to show that we're not OK with the policies that are being passed at the moment," Iliana Panameño, a community organizer for La Comunidad, a nonprofit activist group in Everett, Massachusetts, told NBC Boston.