"The cause may have been the dispersal of a stinging substance, the young people fled and trampled over each other. Sadly, six people died and dozens are injured," the fire service said in a statement on Twitter.
The victims include five people under the age of 18 -- three girls and two boys - and an adult woman who accompanied her daughter to the concert.
Local fire chief Dino Poggiali said that 14 of the injured were in a serious condition, and 40 less serious.
As the crowd panicked, people ran for the three emergency exits, one of which led to a small bridge and the car park.
Grainy mobile phone footage appeared to show part of a walkway collapsing, sending dozens of people tumbling down.
Ironically, those who remained inside the nightclub were unharmed.
"We were dancing and waiting for the concert to start when we smelt this pungent odour," a 16-year-old boy who was taken to hospital told the media.
"We ran to one of the emergency exits but we found it blocked, the bouncers told us to go back."
A pile of abandoned shoes testified to the chaos of the stampede, with survivors and the bereaved weeping outside the nightclub as emergency vehicles tended to the injured, local newspaper Ancona Today reported.
"It can't be! You can't die like that. Wake up Mattia, I beg you," one unnamed woman cried as she knelt by her son's body, the paper said.
Around 1,000 people -- many of them young -- were in the Lanterna Azzurra (Blue Lantern) club for a performance by Italian rapper Sfera Ebbasta, known as "the king of the trap" for his style of Hip Hop.
The Ancona public prosecutor and police are trying to establish how many people were in the venue at the time of the stampede, with media reporting many more people inside than the authorised maximum of 871.
The accident happened at around 1 am (0000 GMT).
The injured were taken to hospital in the nearby town of Senigallia and the most serious on to Ancona.
"You can't die like this at 15," said far-right Interior Minister Matteo Salvini, promising to "find those responsible for these six broken lives, those who through wickedness, stupidity or greed turned a festive evening into a tragedy."
His anti-immigrant League party was to hold a minute's silence during a previously planned rally in Rome on Saturday morning.
Around 30,000 people had gathered in June last year to watch the broadcast from Cardiff, Wales, of the local team Juventus play Real Madrid when pepper spray was shot into the crowd as part of a robbery attempt.
Erika Pioletti, 38, died two weeks later after being left in a coma by the crush. She had gone to watch the game in the square with her boyfriend.
Eight people were arrested on suspicion of causing the scramble by dispersing the irritating substance. (AFP)
(This story has not been edited by Devdiscourse staff and is auto-generated from a syndicated feed.)