Salt Lake City (US), 16 June

A man believed to be part of a peacekeeping team for the "No Kings" protest in Salt Lake City shot at a person who was brandishing a rifle at demonstrators, striking both the rifleman and a bystander who later died at the hospital, authorities said on Sunday.

Police took the alleged rifleman, Arturo Gamboa, 24, into custody on Saturday evening on a murder charge, Salt Lake City Police Chief Brian Redd said at a Sunday news conference. The bystander was Arthur Folasa Ah Loo, 39.

Detectives do not yet know why Gamboa pulled out a rifle or ran from the peacekeepers, but they accused him of creating the dangerous situation that led to Ah Loo's death. The Associated Press did not immediately find an attorney listed for Gamboa or contact information for his family in public records.

Redd said the man who dressed in a neon green vest and was believed to be part of the peacekeeping team fired three shots from a handgun at Gamboa, inflicting a relatively minor injury but fatally shooting Ah Loo.

The gunshots sent hundreds of protesters running, some hiding behind barriers and fleeing into parking garages and nearby businesses, police said in a statement. "That is a gun. Come on, come on, get out," someone can be heard saying in a video posted to social media that appears to show the events.

"No Kings" protests swept across the country on Saturday, and organisers said millions rallied against what they described as President Donald Trump's authoritarian excesses.

Confrontations were largely isolated. They included a driver of an SUV who authorities said struck a woman who was participating in a "No Kings" demonstration and sped away in Riverside, California, east of Los Angeles. The woman had "significant injuries" but was stable, police said, adding that they were still searching for the driver.

Outside the Arizona statehouse, a social media video showed protesters jeering at and then skirmishing with a masked man on Saturday, who eventually pulled out a handgun, causing the crowd to scatter. Another video showed Arizona Department of Public Safety officers taking the man into custody.

The Utah chapter of the 50501 Movement, which helped organise the protests, said in a statement on Instagram that they condemned the rifleman in Salt Lake City, and thanked first responders and "our safety team" for the quick response.

The Utah chapter did not immediately respond to AP questions about the peacekeeping team. It was unclear who hired them, whether they were volunteers or what their training was prior to the event. Redd said that the peacekeepers' actions are also part of the investigation.

The shooter and another person in a neon vest allegedly saw Gamboa separate from the crowd of marchers in downtown Salt Lake City, move behind a wall and withdraw a rifle around 8 pm, Redd said.

When the two men in vests confronted Gamboa with their handguns drawn, witnesses said Gamboa raised his rifle into a firing position and ran toward the crowd, said Redd.

That is when one of the men dressed in the vests shot three rounds, hitting Gamboa and Ah Loo, said Redd. Gamboa, who police said did not have a criminal history, was wounded and treated before being booked into jail.

Police said they recovered an AR-15 style rifle, a gas mask and a backpack at the scene.