A third consecutive day of anti-Trump demonstrations turned violent Thursday night, as protesters began with a rush-hour march and chanting but eventually damaged cars at a dealership and rampaged through the Pearl District shattering business windows into Friday morning.

Police declared the demonstration a "riot" more than three hours after its 5 p.m. start, citing "extensive criminal and dangerous behavior." The bureau said it warned the crowd about the designation, then tweeted that rioting is a class C felony. It later tweeted that 26 were arrested in the demonstration, which lasted into early Friday morning.

Officers in riot gear on numerous occasions used flash-bang grenades and fired rubber pepper ball spheres at the crowd after it refused repeated demands to disperse. Police report officers were "taking projectiles;" protesters were seen throwing objects toward officers, and some fired fireworks in their direction.

The officers didn't move in until after a small fraction of the thousands of protesters became destructive.

Police tweeted that many demonstrators were "trying to get anarchist groups to stop destroying property" and that "anarchists" were refusing to do so. Demonstrators at many points chanted "peaceful protest."

Sgt. Pete Simpson, a Portland police spokesman, told the Washington Post that anarchists "aligned with the Black Bloc groups" infiltrated the peaceful protest while "covered head-to-toe and carrying weapons." He didn't elaborate about what kind of weapons they were allegedly toting, though he said some demonstrators smashed electrical boxes with baseball bats.

Gregory McKelvey, spokesman for the new Portland's Resistance organization, tweeted that the group's protest wasn't responsible for the violence.

A large crowd of protesters -- at least on par with the 2,000 that gathered the night before and estimated as high as 4,000 -- started at Pioneer Courthouse Square in the early evening before taking off on a route that included a stop at the Portland waterfront and trip over the Hawthorne Bridge into Southeast Portland.

It eventually moved into Northeast Portland, where at least 19 cars at Toyota of Portland were vandalized, according to a sales manager.

Protesters then made their way west across the Broadway Bridge and into the Pearl District, where business windows along Northwest Lovejoy Street and elsewhere were smashed.

Car windows getting smashed left and right. #Trumpriot pic.twitter.com/0JyM69bXUb — Mike Bivins (@itsmikebivins) November 11, 2016

About 9:45 p.m., police in riot gear confronted the crowd and shut down the North Park Blocks area. They warned that some protesters were preparing "gas and flares" and that participants should leave for their own safety.

Police said protesters should return to Pioneer Courthouse Square to continue peaceful protest, and those remaining would be arrested.

It was unclear if any arrests had been made by 10 p.m. Most protesters moved in the direction of Pioneer Courthouse Square; a few remained in the park blocks area.

Marchers headed back south into downtown along Broadway. The crowd wound its way through streets, and riot police eventually stopped the protesters after 11:30 p.m. at Southwest 6th and Yamhill Street.

As Thursday became Friday, the standoff continued, with Oregon State Police officers also being deployed at Southwest Yamhill Street and 4th Avenue. Flash bang and less-than-lethal rounds were being deployed as officers advanced to clear the crowd.

By 1 a.m. Friday, the crowd had thinned considerably. Police continued a standoff with a relatively small spattering of protesters, and others farther back. Officers continued to advancing down the street until protesters gradually thinned out.

The crowd by 1:30 a.m. had thinned to the point where no one primary protest group appeared to exist. By 2 a.m. the city appeared to have quieted.

Protesters stayed off freeways in a departure from the previous two anti-Trump demonstrations. Early in the evening, authorities briefly closed freeways as a precaution, TriMet trains and buses were affected, and traffic was disrupted where protesters passed.

Tensions flared at times between protesters and motorists, with police tweeting they received reports of "vandalism and aggressive behavior" in the protest crowd. Altercations included a motorist's windshield being cracked while she tried to navigate through protesters, saying she needed to tend to an emergency.

Some observers reported that a woman apparently was injured after some demonstrators said she was throwing liquid Tide at protesters. A bottle of Tide lay on the ground nearby as demonstrators called for protest medics to tend to the woman, who stayed on the ground for a few minutes.

Trump tweeted mid-Thursday night about the protests that have erupted across the country, calling them "unfair" and prompted by the media.

Just had a very open and successful presidential election. Now professional protesters, incited by the media, are protesting. Very unfair! — Donald J. Trump (@realDonaldTrump) November 11, 2016

A new activist group galvanized by Trump's election was among those demonstrating Thursday in Portland. Dubbed Portland's Resistance and composed of students and youth from protests that took over freeways Wednesday morning and night, spokesman McKelvey said the organization will use anti-Trump efforts to prop up local movements.

He said before the protest that the organization doesn't support destruction and vandalism.

Protest chants included the rallying call of "Not my president," in reference to the newly elected Trump. Some protesters carried signs, among them: "Oppression thrives off isolation. Stand united." "We reject the fascist agenda." "You're fired!"

Halim Byron, speaking from Pioneer Courthouse Square in the early evening, said he decided to join in the latest protest partly because he believes Trump flouts convention -- then flaunts it.

"He does what he wants to, and he's made that a cornerstone of his campaign," said Byron, a 60-year-old Portland resident.

Kaden Burdick, a 20-year-old Portland resident, said in the early evening he sees the election of Trump as a rise of white supremacy and wants to fight against racism, xenophobia and homophobia that Trump's comments appear to have unleashed.

As the larger rally gathered at the Waterfront, a smaller, faith-based group remained back in the small, tiered space in Pioneer Courthouse Square's northwest corner. Leaders from Muslim, Jewish, Christian, Sikh, Buddhist and First Nations faiths spoke to the crowd that lingered long after the larger Trump protest embarked onto downtown Portland's streets.

The leaders put together the event to coincide with the march, said Rev. Michael Ellick, senior minister of First Congregational United Church of Christ.

Ellick, who served as emcee for a parade of speakers, said before the event in a news release: "For just about anybody who isn't white, for anybody who isn't in the top 1 percent and -- you know -- for all women everywhere -- there is a lot of fear right now. So it's more important than ever for the spiritual and moral adults on this planet to show up, speak clearly and get organized."

The audience lingered at least an hour after the larger group departed, mostly listening to speakers and occasionally breaking into song, such as the spiritual, "We Shall Overcome."

They were dwarfed by a 75-foot Douglas fir that had been set up just hours earlier in preparation for the city's annual holiday tree-lighting ceremony on the day after Thanksgiving.

TriMet earlier had warned again that MAX lines and most buses would likely experience significant delays because of the protests -- in Pioneer Courthouse Square, as well as elsewhere around the city, including Mt. Tabor Park and Holladay Park.

TriMet tweeted an appeal for protesters to not block public transit. The agency said it respects the right to peacefully demonstrate, but "if you're out there making your voice heard, please don't disrupt transit service. ... Similarly, we ask that you don't vandalize our equipment."

While the smashing was going on across the river, a group of about 50 people sat on steps or stood waiting to take a microphone along the downtown waterfront as a scattering of votive candles flickered on the ground. They talked about losing a loved one to HIV, supporting Hillary Clinton and even laughed at ways to best challenge Trump.

Two people sat side by side with a sign that said "We shall overcome." A woman with children listened intently.

-- Jim Ryan and Allan Brettman

jryan@oregonian.com; abrettman@oregonian.com

Video by Maxwell Radi/The Oregonian/OregonLive