Lawn chairs stand at the scene of the Fourth of July parade shooting in Highland Park, Illinois on July 4, 2022. (Photo: AFP)

Police arrested a suspect Monday after a mass shooting left six dead at a US Independence Day parade in a wealthy Chicago suburb, casting a dark shadow over the country's most patriotic holiday.

The suspect, identified as Robert Crimo, 22, was taken into custody after a massive manhunt across the town of Highland Park in Illinois where, just hours before, a family-focused July 4 parade celebration had been turned into a scene of death and trauma.

Firing into the holiday crowd from a rooftop with a high-powered rifle, the gunman triggered scenes of total chaos as panicked onlookers ran for their lives, leaving behind a parade route strewn with chairs, abandoned balloons and personal belongings.

Emergency officials said around two dozen people, including children, were treated for gunshot injuries, with some in critical condition.

The Lake County sheriff's office said Crimo was "in custody." Earlier, police had warned that he was armed and "very dangerous." A Chicago musician of the same age and with the same name goes by the stage moniker "Awake the Rapper" online.

The shooting is part of a wave of gun violence plaguing the United States, where approximately 40,000 deaths a year are caused by firearms, according to the Gun Violence Archive website.

And it cast a pall over America's Independence Day, in which towns and cities across the country hold similar parades and people -- many dressed in variations on the US flag -- hold barbecues, attend sports events and gather for firework displays.

"We were getting ready to march down the street and then all the sudden waves of these people started running after, like running towards us. And right before that happened, we heard the pop, pop, pop, pop, pop, and I thought it was fireworks," Emily Prazak, who marched in the parade, told AFP.

- Spectators 'targeted' -

Don Johnson, who attended the parade, said he initially thought the gunshots were a car backfiring.

"And finally, I heard the screams from a block down and people running and carrying their kids and everything, and we ran into the gas station, and we were in there for three hours," he told AFP.

"I've seen scenes like this over and over again on the TV and in different communities, and didn't think it was going to happen here ever," he said.

Police officials said the shooting began at 10:14 am, when the parade was approximately three-quarters of the way through.

"It sounds like spectators were targeted... So, very random, very intentional and very sad," said Lake County Major Crime Task Force spokesman Christopher Covelli.

Five of the six people killed, all adults, had died at the scene. The sixth was taken to hospital but succumbed to wounds there.

Dr Brigham Temple of Highland Park Hospital, where most of the victims were taken, said that it had received 25 people with gunshot wounds aged eight to 85.

He said "four or five" children were among them, and that 16 people were later discharged.

Police said the shooter used a "high-powered rifle," and "firearm evidence" had been located on the rooftop of a nearby business.

"All indications is he was discreet, he was very difficult to see," said Covelli.

A Mexican was among those killed, Mexico's Foreign Minister Marcelo Ebrard said.

"We stand with the Chicago community in its pain and sadness over this tragedy," he tweeted.