The dead were three girls, two boys and a mother who had accompanied her daughter to the disco in Corinaldo, near Ancona, on the central Adriatic coast, Ancona Police Chief Oreste Capocasa said.
The bodies of the victims were found near a low wall inside the building, firefighters spokesman Dino Poggiali told Sky TG24 News.
Survivors said at least one emergency exit door was blocked or did not work. Mr Poggiali said it was too early in the investigation to know if safety violations might have played a role.
He said that when rescuers arrived, all the doors were open.
He said he did not have confirmation from survivors that the use of an irritating spray, like pepper spray, had set off the panic.
Fourteen of the injured were in serious condition, and 40 others less seriously injured, Mr Poggiali said. Some of those with minor injuries were treated and released from hospital, he added.
Firefighters had concentrated on giving first aid to survivors, stretched out on the road outside the club, before starting their investigation, he said.
Italian news agency Ansa said the audience at Italian rapper Sfera Ebbasta's concert at the Lanterna Azzurra nightclub panicked and ran for the exits after someone sprayed a substance similar to pepper spray.
"It was a mess. The bouncers were getting the persons out," one unidentified witness told RAI state radio. "I went out the main door. People fell, one after the other, on top of each other."
A 16-year-old boy told Ansa that fans were dancing while waiting for the start of the show when the stampede erupted. The boy, who was being treated in hospital, said at least one of the emergency exits was locked when he tried to flee.
Authorities did not immediately say how old the victims were, but RAI state radio said the mother was 40, and that about 1,000 people were inside when the stampede began.
Paramilitary police are investigating the cause, in addition to fire officials.