A gunman on a rooftop opened fire on a United States Independence Day parade in suburban Chicago on Monday, killing at least six people and wounding dozens, police have said.

Authorities said 22-year-old Robert E Crimo III was named as a person of interest in the shooting at Highland Park and was taken into police custody on Monday evening after an hours-long manhunt.

The attack occurred at a spot on the parade route where many residents had staked out prime viewing points early in the day for the annual celebration.

It sent hundreds of marchers, parents with strollers and children on bicycles fleeing in terror.