Joey Gibson, head of the group based in Washington state, said he was also in Portland on Saturday night when supporters of President Donald Trump clashed with Black Lives Matter protesters.
Mr Gibson said he arrived at the scene of the shooting shortly after it happened and was briefly corralled in a nearby gas station by angry protesters.
“I can’t say much right now. All I can do is verify that he was a good friend and a supporter of Patriot Prayer,” Mr Gibson said about the victim.
Police issued a plea for videos, photos or eyewitness accounts of the incident which happened about 15 minutes after a caravan of some 600 or so vehicles that formed part of a pro-Trump rally left the city centre.
It is not clear whether the shooting was related to the clashes between Trump supporters and counter-protesters in Portland, which has become a flashpoint in the national Black Lives Matter protests since George Floyd was killed in May, and an increasing centrepiece in Trump’s law-and-order re-election campaign theme.
Police have not released any information about the case but said they were aware of videos on social media that showed the shooting.
Police Chief Chuck Lovell said: “It is still early in this investigation, and I ask everyone to give the detectives time to do their important work before drawing conclusions about what took place.
“If anyone can provide information about this case, I ask them to please reach out to our detectives. This violence is completely unacceptable and we are working diligently to find and apprehend the individual or individuals responsible.”
An Associated Press freelance photographer heard three gunshots and then saw police medics attending to the victim, who appeared to be a white man. The photographer said the man was wearing a hat bearing the insignia of Patriot Prayer.
Police said the man was shot in the chest.
He was not immediately identified, and it is unclear who shot him.
Portland has been the site of nightly protests for more than three months since the police killing of Mr Floyd in Minneapolis.
Many of them end in vandalism and violence, and hundreds of demonstrators have been arrested by local and federal law enforcement since late May.
I'm asking Portland officials, so that's the mayor, that's the governor and that's local law enforcement, to do their job to address any violent activity that is occurring in their streets
In the two hours following the shooting, protesters gathered in the city centre and there was sporadic fighting and vandalism, with 10 people arrested, police said.
The caravan had arrived in the city centre just as a planned protest was getting under way.
The chaotic scene came two days after Mr Trump invoked Portland as a liberal city overrun with violence in a speech at the Republican National Convention.
The caravan marked the third Saturday in a row that Trump supporters have rallied in the city.
Patriot Prayer has a history in Portland and has repeatedly rallied supporters for pro-Trump rallies beginning in 2017.
The group’s supporters periodically cross into Oregon for rallies and marches.
Counter-protesters have squared off with Patriot Prayer and other right-wing groups such as the Proud Boys and the Three Percenters multiple times in the city since Mr Trump’s election.
Mr Trump issued a flurry of tweets and retweets on Sunday, including several that blamed Portland mayor Ted Wheeler for the death and one in which the president appeared to be encouraging his supporters to move into Portland.
“Great Patriots!” Trump wrote as he shared video of his supporters driving into Portland to confront the protesters.
Acting homeland security secretary Chad Wolf blamed local officials for failing “to protect their communities”.
“I’m asking Portland officials, so that’s the mayor, that’s the governor and that’s local law enforcement, to do their job to address any violent activity that is occurring in their streets,” Mr Wolf told CBS.
Mr Wolf said the federal government was prepared to send agents to Portland and other cities to protect federal buildings and assist police.
When federal agents increased their presence in Portland city centre in July, it saw some of the largest protests of the summer, with thousands of people turning out nightly.
The crowds dissipated after the agents withdrew and state police agreed to protect federal buildings for a two-week period.
