Six people are dead and dozens injured in a mass shooting at a 4th of July parade in the wealthy Chicago suburb of Highland Park on Monday, authorities said.

Police said the gunman was still on the loose after he opened fire with a rifle from the roof of a building overlooking the parade route shortly after 10 a.m.

The shooter, who police warned is still “armed and dangerous”, is described as a white male with dark hair, aged 18 to 20 years. He is wearing a blue or white t-shirt, police said.

One witness told WGN TV that he saw the shooter as the gunfire erupted, saying “he was very military style, methodical in the way he was crouched and shooting.”

Other witnesses told the Chicago Sun Times they counted at least 20 gun shots being fired.

“I heard 20 to 25 shots, which were in rapid succession. So it couldn’t have been just a handgun or a shotgun,” local resident Miles Zaremski said.

The rifle was recovered at the scene, cops said.

Footage from the scene showed hundreds of parade-goers — some visibly bloodied — fleeing the parade route as the gunshots rang out just 10 minutes after the parade began.

Chairs, baby strollers and blankets were among the items left behind as people fled.

Witnesses described seeing multiple bloodied bodies covered in blankets on the ground immediately after the shooting erupted.