At least two gunmen opened fire at an arts and music festival in Trenton, New Jersey, early Sunday, injuring 20 people, authorities said.

Shortly before 3am (local time), police received a call about multiple shots being fired during Art All Night, a 24-hour event that draws thousands. One of the suspects, a 33-year-old man, died, while the other was arrested, Mercer County Prosecutor Angelo Onofri told reporters.

Onofri said "multiple individuals opened fire within the venue," a historic building near downtown Trenton. At least 16 of those injured were hit by gunfire, including a 13-year-old boy who is in "extremely critical condition" and three others who are in critical condition, he said. The rest suffered other injuries during the chaos.

Multiple people were wounded early when shooting broke out at the Art All Night festival in Trenton, New Jersey. Photo / AP
Multiple people were wounded early when shooting broke out at the Art All Night festival in Trenton, New Jersey. Photo / AP

Onofri said the shooting appeared to be related to a dispute among people at the event, and police confiscated several weapons. The prosecutor remained tight-lipped about other details of the shooting, including what the dispute was about, who the suspects are, and whether police officers fired the fatal shot.

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"It absolutely could've been worse given the confined space and the number of shots that appear to have been fired," Onofri said, adding that about a thousand people were in the area at the time.

"It's a massive crime scene. There's a lot of people that are injured," he said. "There are a lot of interviews that needed to be conducted."

There are no metal detectors in the building, Onofri said.

Trenton Mayor Eric Jackson condemned what he said was not "just a random act of violence" but a "public health issue" that follows gruesome school shootings that have reignited a nationwide debate over gun control. Just a month ago, a 17-year-old student armed with a shotgun and a pistol opened fire at a high school in Santa Fe, Texas, killing 10 and wounding at least 10.

"All shootings, whether large or small, are a crisis. It's a fact that our cities as well as our suburbs throughout America are experiencing an increase in public shootings and public unrest such as this," Jackson told reporters.

Art All Night is an annual event held in June. More than 900 artists submit their work for display, and attendance surges into the thousands, about 13,000 in 2011, for example, according to the event's Facebook page.

The event was cancelled after the shooting. Event organisers said in a statement that all their staff members, volunteers, artists and musicians were accounted for.

"We know there are a lot of questions and a lot of speculation at this point. We're still trying ourselves to piece this entire situation together ... We're very shocked. We're deeply saddened. Our hearts ache and our eyes are blurry but our dedication and resolve to building a better Trenton community, creativity and inspiration will never fade," the statement said.

Franco Roberts said loud music is usually playing at the event, but that wasn't the case when he and his girlfriend arrived at about 2.30am. They were told that the building would be shut down and turned around to see people "squaring up to fight," Roberts told Homicide Watch Trenton, a community news site.

That's when he heard gunshots.

Multiple people were wounded early when shooting broke out at the Art All Night festival in Trenton, New Jersey. Photo / AP
Multiple people were wounded early when shooting broke out at the Art All Night festival in Trenton, New Jersey. Photo / AP

"Everybody ran toward the door," Roberts said. "And the people fighting got mixed with the crowd that was running and they went out the door shooting."

Irvin Higgenbotham, who comes to the event every year, said he was walking with his bike when he heard gunfire. He had been shot in the leg.

"I was like, pow, pow pow, and then I was laying down on the ground," he told NJ.com.

Hours later, Higgenbotham, with crutches and his left leg completely covered in bandages, went back to the crime scene to find out what had happened.

Krystal Knapp, who said she was a volunteer at the event, wrote on Facebook that she first noticed some "commotion" behind her, saw people "pushing and shoving each other,' and heard gunshots. People began running toward the door, and Knapp said she was knocked down near the exit.

"A kind woman pulled me up and over to the side and told me to stay down with her," Knapp said. "A woman three feet from us was shot in the leg."

There were off-deputy police officers working security at the event all night, Knapp said, "but some idiot decided to pull out a gun and harm the best night of the year in Trenton."

"I hope this doesn't ruin Art All Night," she added. "That would be letting violence win."