DENVER — One person was fatally shot in Denver on Saturday against the backdrop of opposing rallies between far-right and far-left activists.

The circumstances were still unclear Saturday night, but a local television station reported that a security guard it had hired was being detained in connection with the shooting.

The shooting happened at 3:37 p.m. local time near the courtyard of the Denver Art Museum as the protests were winding down, officials said.

A video on Twitter and a Denver Post photograph captured the chaotic moments leading up to the gunfire.

A man wearing a “Black Guns Matter” T-shirt is seen on the video loudly arguing, in explicit language, with several men. One of the men moves out of view of the camera, and seconds later, he is shot.

The camera captures him falling to the ground. The man in the T-shirt was not involved in the shooting. The Post photograph shows the victim spraying Mace or a similar substance right before he is shot.

Another video shows several police officers rushing to the scene, shouting, “Drop the gun!” A man can be seen getting down on the sidewalk as officers surround him.

Both the victim and the gunman were white men, Joe Montoya, chief of the Police Department’s investigations division, said at a news conference on Saturday evening. The victim was taken to a hospital, where he later died, he said.

They were not publicly identified by the authorities, but the police said on Twitter that the gunman was a private security guard with no affiliation with antifa, a loosely affiliated movement of activists who share far-left anti-fascism views and which had come out to protest in Denver on Saturday.

The police did not announce the filing of any charges on Saturday.

ImagePolice officers took the man who fired the shot into custody. A local television station said he was a private security guard hired to protect its journalists.
Credit…Helen H. Richardson/The Denver Post, via Associated Press

A Denver news channel, 9News, reported that a station producer and a contractor had been taken into custody.

“A private security guard who was hired by 9News is the suspect detained by DPD,” the station reported. “It has been the practice of 9News for a number of months to hire private security to protect staff at protests.”

The security guard was contracted through Pinkerton, Mark A. Cornetta, president and general manager of 9News, said on Saturday night. He said the producer had been released.

A representative for Pinkerton could not be immediately reached late on Saturday night.

Chief Montoya said a canister of Mace and two guns had been recovered at the scene.

Tensions were high all afternoon. The Denver police, and personnel from the sheriff’s offices of Arapahoe and Douglas Counties, were present at the protests before the shooting.

Asked why there had been such a heavy police presence, the chief said it was because “we had two groups with opposing views.”

Chelsea Jacobs, a member of the Denver Communists, said that Saturday’s event, which the group called a Black Lives Matter-antifa soup drive, had been organized in response to a “Patriot Muster” that the group saw being advertised about a week ago.

The “soup rally” was meant to be ironic in response to comments that far-left protesters throw soup cans, she said.

Ms. Jacobs said that she had been at the north end of the park, where the police had moved the group she was with, and that the shooting had happened at the south end of the park.

Demonstrators from the right-wing group filmed the Denver Communists over a temporary fence that separated the two sides, but the groups did not physically clash, she said.

A tweet promoting the Patriot Muster called for all “patriots” to “Stand up. Show up.”

“It’s time to Roll,” an activist, John Tiegen, wrote on Twitter. “BLM, Antifa, and related Marxist associations are assaulting everything we love and cherish about this country.”

Opposing groups have faced off, sometimes violently, around the country over the last few months as racial unrest and a looming election have ignited protests across the country.

In Denver, businesses near where Saturday’s protests were held were warned about the potential for violence after clashes earlier this summer. Protesters with the Party for Socialism and Liberation disrupted a pro-police rally in July, and the two groups engaged in fist fights, CBS 4 reported.

On Aug. 29, a man affiliated with a right-wing group was shot and killed in Portland, Ore. The gunman, an antifa supporter, was killed by officers attempting to arrest him.

Two men were shot dead in August while protesting the police shooting of a Black man, Jacob Blake, in Kenosha, Wis. A 17-year-old, Kyle Rittenhouse, has been charged in their deaths.

In Austin, Texas, in July, Garrett Foster, a Black Lives Matter supporter, was shot dead while carrying an AK-47 rifle during a protest. A lawyer for the man who shot him said he was acting in self-defense.

There have also been instances of people driving into demonstrators. In July, Summer Taylor, 24, was struck and killed while participating in a march on a closed section of Interstate 5 in Seattle.

Bryan Pietsch reported from Denver, and Concepción de León from New York.