At least one person was killed and 22 others injured when gunfire broke out early Sunday at a massive outdoor Juneteenth celebration in the Chicago suburb of Willowbrook.

About 200 teens and young adults were partying in a strip-mall parking lot when dozens of gunshots rang around 12:30 a.m., sending the panicked celebrants fleeing for their lives, according to WSL-TV.

“We were all just out, and next thing you know, shots just kept going off,” witness Craig Lotcie told the local ABC affiliate. “Everybody ran, and it was chaos.”

Authorities said some of the victims were struck while trying to flee the mayhem. At least one person died in the violence, and fire officials told WBBM-TV that two of the injured were in critical condition.

Most of the others hurt suffered graze wounds, authorities said.

It wasn’t clear how many gunmen were involved in the bloodshed, but there were “multiple weapons” involved, the DuPage County Sheriff’s Office told The Post in a statement.

Cops have yet to release a motive. 

Police said they had been monitoring the party since about 6 p.m. Saturday but were called away from the scene around 12:25 p.m. when a 911 call came in reporting a nearby fight.

As officers drove off, they heard dozens of gunshots coming from the party and hurried back. By the time they arrived, the gunfire had ended.

Numerous videos showed pandemonium as heavily armed officers descended on what had been jubilant just moments before.

“It was supposed to be like a Juneteenth celebration,” witness Markeshia Avery told the local ABC affiliate. “We just started hearing shooting, so we dropped down until they stopped. They just kept going. After that, we literally scattered away.”

Cops remained at the scene into Sunday afternoon, with caution tape cordoning off the area, which was still littered with trash and debris left behind by the panicked partygoers.