A suspected gunman is dead and at least 22 people are injured after shooting erupted at an all-night arts and music festival in the United States late Sunday night (AEST), sending people running over each other in the scramble to safety.

Two gunmen opened fire at the Art All Night Trenton festival in New Jersey,  authorities said. One suspect was killed and 17 people sustained gunshot wounds.

Of people treated for gunshots, four of them, including a 13-year-old boy, remained in critical condition Monday morning, Mercer County Prosecutor Angelo Onofri told the Associated Press.

The shots rang out around 2.45 am local time at the festival, which showcases local art, music, food and films.

Mr Onofri said a 33-year-old man was killed, apparently by police, and the second suspect was in custody. He said a “neighbourhood beef” was behind the shooting.

On Monday, there was crime scene tape surrounding the site of the old warehouse that now shares a parking lot with a supermarket, bank and laundry facility.

Police are also investigating an attempted carjacking that occurred in an alley close to the crime scene. Mr Onofri said police were working to determine if it was connected to the shooting.

Art All Night Trenton began Saturday afternoon and was scheduled to conclude on Sunday afternoon. The remainder of the festival has been cancelled.

Gennie Darisme was getting ready to leave the festival around 2.45 am local time when she heard shots and saw people running.

“There were people trampling other people, cars hitting other cars,” she told AP.

When she was walking back to her car after the shots stopped, Ms Darisme said she saw someone bleeding on the ground, in handcuffs.

“People were running to him, trying to see his face, to see if he’s a family member or a friend,” she said.

A spokeswoman for St Francis Medical Center in Trenton said 10 victims were being treated there for minor injuries. She said they were both men and women, ranging in age from 17 to 48.

They were in various stages of being released, she said.

One man with a gunshot wound was transferred to Cooper University Hospital in Camden.

Capital Health Systems spokeswoman Kate Stier said they have “at least 16″ patients there, including the 13-year-old boy in critical condition. That total may not include people treated and released.

Trenton Mayor Eric E Jackson said the violence could not be “discarded as just random violence”.

This is a public health issue,” he said.

-With AAP