The collapse in Wildwood happened at around 6 p.m. during the annual New Jersey Firemen's Convention, which attracts thousands of current and former firefighters to the resort town.
Cape May Regional Health System said 21 people were taken there, at least three of them children. Eleven patients had been released by 10 p.m., including all the children who were admitted, hospital spokeswoman Susan Staeger told The Associated Press. AtlantiCare Regional Medical Center in Atlantic City said at least one additional person was taken there.
The conditions of the hospitalized victims were not immediately clear, but no deaths were reported.
Witness Joann Devito said she was sitting on her deck across the street when she saw the collapse.
"I heard this noise, so I turned and looked and saw the whole thing caving in," she said. "I saw two women running and screaming. It was horrible."
Photos and video on social media showed firefighters trying to lift a piece of decking. It was unclear how many people were on or under the decks at the time, or how many were firefighters, but authorities said those who were trapped were quickly removed.
Images showed multiple levels of decking ripped away from the building and reduced to beams and splintered wood. The decks appeared to have been topped by a fourth-floor overhang, which looked to have remained intact. The second and third levels appeared to have pancaked onto a first-floor deck. The overhang looked intact.
Wildwood Fire Chief Daniel Speigel said authorities would not make a statement Saturday amid attempts to clean up and investigate. More information is likely on Sunday, he said.
The Associated Press contributed to this report.