Authorities said organizers had sold far too many tickets for the space.
Witnesses said people were crushed to death after falling from a small wall as they attempted to flee.
Later, Premier Giuseppe Conte, who visited the scene, said the disco had three rooms but inexplicably only used one for the concert, and it only holds 469 people. He identified those killed as Asia Nasoni, 14; Daniele Pongetti, 16; Benedetta Vitali, 15; Mattia Orlandi, 15; Emma Fabini, 14; and 39-year-old Eleonora Girolimini.
Five of the victims were minors and the sixth victim was the mother of one of them, ANSA reported. "It was way overcrowded and alcohol abounded".
The injured were rushed to hospitals in Senigallia and Ancona. Some 40 others were less seriously injured, Poggiali said. The Lanterna Azzurra club was packed at the time and numerous injured suffered crushing wounds. The incident occurred at a Sfera Ebbasta concert on the east coast.
He said he would cancel his promotional events over the coming days out of respect for the dead and injured.
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The rapper added he wanted everyone to "to stop and think how unsafe and stupid it is to use pepper spray in a discotheque".
Teenagers were out due to the Immaculate Conception holiday in Italy in which high schools are closed.
He said that when rescuers arrived, all the doors of the discos were open.
"We will find those responsible for these six broken lives, those who out of malice, stupidity or greed have turned a party night into a tragedy", he added in a statement.
Italy's head of state, President Sergio Mattarella, demanded "full light be shone on what happened, ascertaining any responsibility and negligence". "Therefore, safety must be assured with particular care in crowded meeting places, through rigorous controls", Mattarella said.
In 2017, a woman died and 1,500 were injured in a crush during the broadcast of a Champions League football match final in Turin when rumours of a bombing sparked panic.