Chain-reaction brawls involving up to 2,000 people erupted in one of Kentucky's largest malls Saturday night, forcing the entire mall and businesses in the surrounding area to shut down, police said.

The hours-long chaos at Mall St. Matthews began about 7 p.m. ET when the six St. Matthews police officers assigned to the mall for the holiday season began responding to "disturbances," said Officer Dennis McDonald, a police spokesman.

"As they were responding to those disturbances, others were breaking out. ... Disturbances started to feed on themselves." McDonald said. "They were just overwhelmed with a number of calls for service and reports of disorder."

The officers on duty at the mall called for backup, and 50 officers from five different agencies responded, according to police.

"It was a series of brawls" involving 1,000 to 2,000 people ages 13 to their early 20s, McDonald said, adding that "the entire mall" was affected.

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About 8 p.m., authorities started to advise stores in the mall to close their doors, but those involved in the brawls were refusing to leave. "Businesses were in the process of closing their doors, steel grates, and you had juveniles that were not allowing businesses to close up — [they were] climbing on the grates," McDonald said.

"This was a riot," McDonald added. "It was crazy."

Police received reports of shots fired within and outside the mall, but investigators haven't confirmed those reports, McDonald said.

Restaurants and shops surrounding the mall also closed their doors as the teens and young adults flooded out, McDonald said.

"It took about an hour and a half, close to two hours, before things were calm," McDonald said. Officers "maintained a presence" until 1 a.m., McDonald said, adding: "We're all tired."

But no one was arrested, and only minor injuries were reported, McDonald said. No officers were injured.

"Our officers, they showed great restraint," McDonald said. "Officers were focused on dispersing crowds and keeping them moving."

McDonald said investigators haven't determined what sparked the outbreak of violence, but they don't believe it was planned.

Mall St. Matthews didn't immediately respond to a request for comment from NBC News. The mall returned to normal business hours and reopened at 11 a.m. Sunday, according to the mall’s security office.

St. Matthews, a suburb of about 18,000 people outside of Louisville, is a normally quiet city that sees no more than two murders a year, McDonald said. "I've been a police officer 33 years, and I haven't ever seen anything like this before," he said. "We always plan for worst-case scenario, but this exceeded that."