Two people were taken into custody after a man was shot and killed at duelling ideological protests between "patriots" and left-wing "BLM-Antifa" in the US on Saturday.

The shooting took place outside the Denver Art Museum in Colorado, about 3.37pm local time, according to Denver Police Department.

It followed a "verbal altercation", Division Chief Joe Montoya said during a press briefing on Saturday night. Two guns and a can of mace were recovered at the scene.

Police said the victim was transported to hospital at 3.49pm, but had died an hour later.

"This shooting is now being investigated as a homicide," Denver Police said in a statement. "Updates will be posted as information comes available."

In a follow-up statement the Denver Police Department said they had taken two suspects into custody in the early hours of Sunday morning, however they later announced one had been released.

Video shows officers arresting one suspect outside the museum at the end of the rally.

In another video shared online, a gunshot can be heard as witnesses rush to the scene. Two men can be seen putting their hands in the air as they're surrounded by officers.

The incident occurred during two ideologically opposed protests, both held at Denver's Civic Center Park, nearby the Denver Art Museum.

One protest called "BLM-Antifa Soup Drive" had been hosted by the Denver Communists, while a "Patriot Muster" rally was organised for the same location, asking followers to "stand up" and "show up".

Videos showed heated arguments between protesters before the incident.

It's unclear if the suspects or the victim had been affiliated with any of the groups, Chief Montoya said, according to US ABC News.

"There was a large presence because we had two groups with opposing views, and we know that can always get very tense, and there's always potential for violence," he said.

Denver Police subsequently said in a statement that "further investigation has determined the suspect is a private security guard with no affiliation to Antifa".

A journalist from the Denver Post, who claimed to have witnessed the incident, said a man affiliated with a "Patriot Rally" sprayed mace in the face of one of the left-wing protesters, who then shot him.

The Denver Post updated its story after "authorities called into the question the accuracy of that journalist's report concerning the affiliation of the shooter", the editor's note said.

9News Denver identified the two arrested as one of its producers and a private security guard hired by the station.

The producer was "released after DPD said it was determined they were not involved in the incident", the station said.

Originally published as Man shot dead as 'Patriots', Antifa clash