The melee started when attendees tried to forcefully enter the Summer Jam concert at MetLife Stadium in East Rutherford, authorities said.
"Security personnel at one of the entrance gates ... were confronted by crowds attempting to illegally enter the sold out Summer Jam concert by climbing over fences and forcing their way through security personnel," New Jersey State Police said.
Troopers on site asked for extra backup to help maintain order, Sgt. Gregory Williams said. Witnesses said authorities arrived in riot gear.
"We have the riot gear all the time and we were ready in case something was to happen and we had to use it," he said.
But some witnesses provided a different account.
Jerry Arnold, who was at the concert but did not get in, said attendees got upset because it was taking too long to go through security.
"Security was being extremely slow, and people were waiting to be let in," he said. "I had a ticket. Everyone had ticket, but security was being very slow and everyone got agitated."
Officers with shields used mace to disperse the crowd, causing more chaos and delaying the concert by hours, according to witnesses.
"People were falling down, trampling onto each other," Arnold said. "The concert started at 6 p.m. and at 9 p.m., people still weren't getting in."
That got the crowds more livid.
"People started getting really rowdy, saying like, "I paid for my ticket, I want in, now!" said Sarah Duvivier, another concertgoer.
Fights broke out as crowds pushed through a metal barrier. Others hurled bottles at officers, she said.
Duvivier and Arnold said things got so chaotic, they left without attending the concert.
The Summer Jam concert featured artists such as Kendrick Lamar, Big Sean, Chris Brown and Trey Songz.
Several arrests were made, Williams said, but he did not provide a number.