Police are responding to a shooting at a July 4 parade in a Chicago suburb, officials said.

Authorities have not officially reported any casualties, but witnesses described seeing bloodied bodies apparently covered with blankets.

The Chicago Sun-Times reported that the parade began around 10am on Independence Day in the US but was suddenly halted 10 minutes later after shots were fired.

Several witnesses told the newspaper that they heard gunfire.

Hundreds of parade-goers – some visibly bloodied – fled the parade route, leaving behind chairs, pushchairs and blankets.

A Sun-Times reporter saw blankets placed over three bloodied bodies.

Police told people: “Everybody disburse, please. It is not safe to be here.”

Video shot by a Sun-Times journalist after the gunfire rang out shows a band on a float continuing to play as people run past, screaming.

A photo posted to social media appeared to show pools of blood near upturned chairs in downtown Highland Park.

Debbie Glickman, a Highland Park resident, said she was on a parade float with co-workers and the group was prepared to turn onto the main route when she saw people running away from the area.

“People started saying ‘there’s a shooter, there’s a shooter, there a shooter’,” Ms Glickman told the Associated Press. “So we just ran. We just ran. It’s like mass chaos down there.”

She did not hear any noises or see anyone who appeared to be injured.

“I’m so freaked out,” she said. “It’s just so sad.”

The Lake County Sheriff’s Office said on Twitter that it is assisting Highland Park Police “with a shooting in the area of the Independence Day parade route”.

The sheriff’s office directed an AP reporter to contact Highland Park Police. The Police Department said no one was immediately available to discuss the incident.

City leaders said on Twitter: “Police are responding to an incident in downtown Highland Park. Fourth Fest has been cancelled. Please avoid downtown Highland Park. More information will be shared as it becomes available.”

The city said on its website that the parade would feature floats, marching bands, novelty groups, community entries and other special entertainment. A children’s bike and pet parade also was scheduled to start 30 minutes before the main parade.