PORTLAND, Ore. — One person was shot dead on a Portland street Saturday night during a series of confrontations between members of a 600-vehicle caravan in support of President Trump and counterprotesters who met them in this riverside city, an intensification of the conflict over race and criminal justice that has roiled American communities during a summer of illness and anguish.

Occurring in the aftermath of back-to-back political conventions, and following racial upheaval in Kenosha, Wis., that left two protesters dead, the armed hostilities signaled how political strife is manifesting itself in violence and further scarring this country as it moves toward the fall’s presidential contest.

Police here said they are investigating the shooting as a homicide but warned against forming conclusions about what had occurred because so much was still unclear Sunday, almost 24 hours after the lethal encounter. Police did not release information about a potential suspect or the victim, though it appeared from videos of the scene that they were on opposite ends of the skirmishes — the victim allied with the pro-Trump rally and the shooter congregating with Black Lives Matter counterprotesters before opening fire.

The man who was shot, falling to the pavement in the downtown district near where the opposing groups had clashed throughout the evening, was wearing a hat with the insignia of Patriot Prayer, a far-right group based in the Portland area. Its leaders publicly mourned the loss of a man they identified as Aaron J. Danielson, saying he had “such a huge heart.”

Family members and friends of Danielson, who went by “Jay,” declined to comment when reached Sunday. Kevin Allen, a spokesman for the Portland Police Bureau, said authorities were waiting for an autopsy to release information about the victim.

Trump, in a barrage of tweets on Sunday, seized on the killing as a cudgel against Democratic leaders in Portland, a city where his administration has deployed federal officers against the wishes of local officials. He portrayed the urban mayhem, which has frequently punctuated his fourth year in the White House, as a case for his reelection pitch of “law and order.” And he heaped praise on the pro-Trump activists who had descended on Portland, calling them “GREAT PATRIOTS!”

Portland Mayor Ted Wheeler turned the blame back on the president, saying Trump had “supported and energized” provocateurs who had come to the city to create chaos.

“He encouraged them to come into our community, and previously he has actually encouraged, or at least tacitly approved of, violence,” said the Democratic mayor, who vowed to defend “to the death” the right to nonviolently demonstrate. “The violence, however, is the problem.”

Joe Biden, the Democratic presidential nominee, also directed blame at the president, saying in a statement on Sunday afternoon that “Donald Trump has been president for almost four years. The temperature in the country is higher, tensions run stronger, divisions run deeper.”

As he faulted Trump, the former vice president also forcefully denounced the bloodshed in Portland.

“I condemn this violence unequivocally,” he said. “I condemn violence of every kind by anyone, whether on the left or the right.”

In a video recording of the scene, which captures the gunfire from across the street, the sound of men shouting can be heard before a thick cloud of what appears to be mace surrounds them. Shots are fired. Several people run, before others crowd around the man who had stumbled and collapsed face down on the pavement.

“I heard the yelling, and I turned, and the first thing I saw was the bear mace being sprayed, and then right on the heels of that were the gunshots,” said Justin Dunlap, a lighting technician from nearby Vancouver, Wash., who recorded the widely circulated video and broadcast it live on Facebook.

Dunlap, 44, said when the gunshots were fired, “the guy who had sprayed the bear mace turned and took three or four steps and then went face down. While he was doing that, the two that he had bear maced ran back in my general direction and kind of went around the corner.”

Initial accounts from law enforcement depicted a similar scene.

“Portland Police officers heard sounds of gunfire from the area of Southeast 3rd Avenue and Southwest Alder Street,” the Portland Police Bureau said in a statement. “They responded and located a victim with a gunshot wound to the chest. Medical responded and determined that the victim was deceased.”

Other video from the scene captures sounds of a pursuit, with men referring to mace and shouting, “We got a couple right here.”

The same video shows a man coming to the aid of the shooting victim and angrily pushing away black-clad protesters, even though they expressed a desire to help. An onlooker calls out, “Do you need an ambulance?”

Police and emergency medical vehicles surrounded the shooting victim moments after he was hit. That in turn amplified tensions among protesters, as police attempted to secure the area in a busy part of the city, which has seen nightly demonstrations for three months.

The stated aim of Patriot Prayer, organized in 2016 to bring pro-Trump rallies to liberal bastions, is to “liberate the conservatives on the West Coast.” Its leader, Joey Gibson, was swarmed later Saturday night as he ran through the streets, following the fatal episode involving the man wearing his organization’s logo.

“All I can do is verify that he was a good friend and a supporter of Patriot Prayer,” Gibson wrote in an email to The Washington Post. He said he would be making an additional statement later Sunday or on Monday.

It remained unclear what exactly precipitated Saturday’s fatal shooting, as Trump supporters shot paintballs and pepper spray from their trucks and activists burned Trump flags and lobbed rocks and other projectiles at the moving vehicles. One person who claimed to be an organizer of the pro-Trump event said they had instructed participants to stay on the parade route and did not know why the person who was shot had deviated or what had transpired.

“At this point we are simply trying to get people to stay calm so this does not escalate,” the organizer said. “We’d like to bury our dead.”

Numerous rallygoers had gone off the designated parade route, engaging in fights with counterprotesters who were shouting at passing vehicles and using megaphones. The shooting took place several blocks from the route.

Police Chief Chuck Lovell said his bureau’s plans were thrown into disarray when members of the caravan departed from the planned route and made for the core of downtown. Those vehicles sped past law enforcement, and there was little if any attempt to stop them from entering the city.

The episode came as a climax to a turbulent week, which opened Aug. 23 when police in Kenosha, Wis., shot a Black man, Jacob Blake, multiple times in the back. The encounter spurred renewed protests against racial discrimination and police violence. In the ensuing turmoil, militia groups flocked to the city, and a 17-year-old, Kyle Rittenhouse, was charged in the fatal shooting of two protesters there.

Protests against police brutality have become a regular fixture in some cities, including Portland, since the death in police custody of George Floyd in May. The turmoil has come in the midst of an explosive election season, during which Trump has exploited urban unrest to amplify his message of “law and order,” and the demonstrations have assumed partisan overtones.

Earlier Saturday in downtown Portland, skirmishes between pro-Trump rallygoers and Black Lives Matter activists left multiple people injured. The sparring groups threw punches at one another and hurled debris among vehicles. Some broke into open fighting in the streets. Trump supporters in trucks were at one point blocked in by Black Lives Matter activists and began exiting their vehicles, precipitating the violence.

Blood was streaming down the face of one Trump supporter who had challenged an activist to a fight.

Tony Bartell, 26, of Vancouver, Wash., said one of the Trump rallygoers punched him after jumping out of a vehicle. Bartell had photographed his license plate, agitating a man.

“While he’s in my face and I’m recording him, someone else comes up behind me and smacks my phone on the ground,” he said. Bartell said he was hit in the face and shaken up.

The Black Lives Matter activists initially blocked traffic onto the highway as some shouted, “Just go home!” and “Don’t come to our city!” Others urged restraint, yelling, “Don’t give them a reason!” They apparently were trying to ensure a vehicle would not charge into the activists.

A small fight broke out at the head of the on-ramp before police arrived moments later and separated the groups, which had gathered earlier in the day at a shopping center just outside Portland. Police were seen making multiple arrests.

The Trump supporters, some armed and many flying “Trump 2020” and pro-police flags from their trucks, were set to follow a highway route around the city, according to a map posted on social media. The groups were separated after skirmishes broke out, and they began hurling obscenities at each other from opposite sides of the highway.

A Facebook event for the gathering, called “Trump 2020 Cruise Rally in Portland,” had drawn more than 13,000 respondents, with nearly 3,000 users saying they had attended. Organizers of the event made multiple posts asking participants to follow the designated route. “We would like to get some fellow American patriots together for a Trump 2020 cruise rally to support our great president,” the description stated.

But the online conversation shifted from political support for the president and predicted political violence.

“You will be red blood to the blue sharks!” one user wrote on Friday, seeming to anticipate clashes in the liberal city. On Sunday, other users were eulogizing the shooting victim as a “fallen patriot.”

The digital dimension of recent skirmishes has brought new scrutiny to Facebook, whose chief executive, Mark Zuckerberg, acknowledged last week the company had erred by not removing an event listing that encouraged armed civilians to defend the streets of Kenosha.

The deadly denouement of Saturday’s events in Portland marred efforts by Dunlap, the lighting designer, to broadcast a different image of recent protests, one not defined by violence.

First brought to the demonstrations in July by his daughters, the 44-year-old said, he had returned regularly, in a white helmet and orange shirt, to document peaceful assemblies, posting lengthy videos on Facebook. A native of Oklahoma, he called his visits “democracy field trips,” designed to “show people back home that Portland wasn’t actually all on fire.”

He left the city shortly before midnight Sunday, having given his statement about the shooting to homicide detectives.

Instead of a refuge for peaceful protest, Portland had become a study in the effects of extreme polarization, said Marc S. Rodriguez, a historian at Portland State University. The divisions, he said, were not just over political alternatives but alternative accounts of reality, enabled by a fractured media environment.

The labeling by Trump and his allies in conservative media of all protesters as “anarchists” and “Biden voters,” Rodriguez said, “is reminiscent of what we see in dictatorships." And, he added, “self-styled antifascists have responded with hard-core tactics.”

“The question,” Rodriguez said, "is how does the country heal from what has a been an extremely disruptive year?”

Stanley-Becker reported from Washington. Desmond Butler, Katie Shepherd and Julie Tate in Washington contributed to this report.