Updated: 10.45am

At least six people have been killed and dozens injured after a gunman opened fire during a 4th July parade near Chicago.

Police have arrested a suspect following a brief car chase after earlier announcing they were searching for a 22-year-old suspect, identified as Robert (Bobby) E Crimo III.

The incident unfolded at about 10.10am local time when a gunman is understood to have opened fire from a rooftop in the community of Highland Park, about 40 kilometres north of Chicago, during the parade. Authorities said a high-powered rifle had been recovered from the scene.

Highland Park Mayor Nancy Rotering said in a statement that it was a "terribly difficult day, and the bloodiest that we have experienced in Highland Park".

"Our community, like so many before us, is devastated," she said. "It’s impossible to imagine the pain of this kind of tragedy until you’re confronted with it."

"Gun violence - 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."

He said President Joe Biden has called to offer condolences and federal resources.

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Illinois Governor JB Pritzker said there were "no words" to describe the loss.

"There are no words for the kind of evil that shows up at a public celebration of freedom, hides on a roof and shoots innocent people with an assault rifle," he was quoted as saying on CNN. "There are no words I can offer to lessen the pain of those families who will no longer associate the 4th of July with the celebration, but instead with grief."

In a statement on Twitter Biden said he was "shocked by the senseless gun violence that has yet again brought grief to an American community this Independence Day".

"As always, we are grateful for the first responders and law enforcement on the scene. I will not give up fighting the epidemic of gun violence."