Police arrested a suspect on 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.

Robert Crimo, 22, was identified as a “person of interest” and became the target of a massive manhunt across the town of Highland Park in Illinois, where a rooftop gunman with a high-powered rifle turned a family-focused July 4 parade celebration into a scene of death and trauma.

Firing into the holiday crowd, the shooter caused scenes of 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.

After a brief car chase, Crimo was taken into custody “without incident,” Highland Park police chief Lou Jogmen told reporters.

Earlier, police had warned that he was armed and “very dangerous”.

Crimo bills himself as a musician, and goes by the online moniker “Awake the Rapper”.

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.

In another July 4 shooting, two police officers were wounded when they came under fire during a fireworks display in Philadelphia, local officials said.

CBS News aired video taken from a high-rise building showing crowds fleeing in panic as fireworks burst in the sky.

Philadelphia police commissioner Danielle Outlaw said both officers had been released from hospital after receiving treatment, and that authorities were still investigating the exact circumstances of the incident.

In Highland Park, Emily Prazak, who marched in the parade, described the mayhem.

“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,” Prazak told AFP.

She added: “This is the day that we celebrate our country. This is also a day that our freedom got stolen from us — because many of us residents here, in this building even, we're all locked down.”

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, died at the scene. The sixth was taken to the hospital but succumbed to wounds there.

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

“Four or five” children were among them, he said, 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.

US media reported that Crimo's online postings included violent content that alluded to guns and shootings. His YouTube and other social media accounts were not viewable on Monday night.

One YouTube video posted eight months ago features images of a young man in a bedroom and a classroom along with cartoons of a gunman and people being shot, the Chicago Tribune reported.

A voice-over says “I need to just do it.”

It adds: “It is my destiny. Everything has led up to this. Nothing can stop me, not even myself,” the newspaper said.

311 mass shootings so far

In a statement, Highland Park Mayor Nancy Rotering said the town had been “devastated” by the shooting.

“A mass shooting such as this casts a much wider net of agony than what the public is typically exposed to; it's a crisis that devastates entire families and communities in a single moment and we know will take time to heal,” she said.

US President Joe Biden voiced his shock and vowed to keep fighting “the epidemic of gun violence” sweeping the country. “I'm not going to give up,” he said.

Last week, Biden signed the first significant federal bill on gun safety in decades, just days after the Supreme Court ruled that Americans have a fundamental right to carry a handgun in public.

The deeply divisive debate over gun control was reignited by two massacres in May that saw 10 Black supermarket shoppers gunned down in upstate New York and 21 people, mostly young children, slain at an elementary school in Uvalde, Texas.

According to the Gun Violence Archive, there have been 311 mass shootings carried out in the US so far in 2022 — including at least five other incidents on July 4.