The wave of violence broke out as nearly 20,000 people left Newark's Prudential Center after a sold-out concert Saturday by the band the Red Hot Chili Peppers.
Three teenagers and two adults were targeted, including a 23-year-old Pennsylvania man, who suffered a fractured eye socket, police said.
Newark Police Director Samuel DeMaio said the suspects appeared to be a band of 10 or 15 "thugs." No arrests have been made.
DeMaio told The Star-Ledger, which first reported the incident, that the suspects appeared on video surveillance to be laughing during the brutal assaults.
"I think the taking of the cellphones was secondary. I think the main focus was to assault them," DeMaio said. "Some of the kids were laughing through the assaults."
A New Providence father was left unconscious and bleeding in front of his 14-year-old son, according to a police report obtained by the newspaper, which also reported that the chaotic scene prompted commanding officers to issue a "citywide alert" for help.
Police officials said the attacks happened in the span of a few minutes, with the group attacking five people in three separate incidents along the main city thoroughfare of Broad Street.
A spokeswoman said Mayor Cory Booker viewed the incident as "isolated" and "unfortunate."
"The city continues to provide police protection around the arena that goes above and beyond what is practiced at other arenas across the country, and still stands as one of the best guarded facilities in the city," Booker spokeswoman Anne Torres said.
The mayor last month announced a reduction in police at the arena, denying the move was in response to an arbitration ruling that sided with the New Jersey Devils - the arena's anchor tenant - in a bitter, long-running financial dispute over revenue-sharing. But Torres said the reduction had not been in effect Saturday, adding there was a full contingent of more than 20 officers patrolling the arena and its surroundings. Torres said police presence has only been reduced at smaller events, from 20 officers to 14, but Saturday's sold-out concert warranted full staffing.
Prudential Center officials praised the work of the Newark Police Department and wished the victims a speedy recovery.
Detective Sgt. Ronald Glover said it was the only the second violent incident at the arena, which has had more than 8 million visitors, since it opened in 2007.
In March 2009, a gay couple who had just left a Britney Spears concert at the arena were attacked and robbed by a marauding group of young people, according to police. Victim Joshua Kehoe said at the time that he and his boyfriend, Bobby Daniel Caldwell, whose jaw was broken in two places, had no doubt the attack was motivated by bias.
The couple were walking to their car when they were approached by a group of about 15 youths. Police said at the time the group had assaulted and robbed four women a block away before encountering the couple.
DeMaio said Wednesday that members of the public should not be afraid to attend arena events.
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