The suspected use of a pepper spray-like substance is thought to have sparked the chaos at the venue in the town of Corinaldo, near Ancona in central Italy, the fire service said in a statement on Twitter.
|Ten people are in a critical condition with 120 injured in total. (AFP)|
But carabinieri police spokesman Cristian Carrozza later said they were still "trying to ascertain what had caused the youths to rush out", adding: "There are three exits and one of these was used."
The victims include three girls and two boys aged between 14 and 16 and a 39-year-old woman who accompanied her daughter to the club.
Some 10 people are in a critical condition, doctors said, with at least 120 injured in total.
"Around 1,400 tickets were sold for the concert and the place could only accommodate 870 people," said Monica Garulli from the Ancona prosecutor's office.
The exit used led to a small bridge and the car park. The force of the fleeing crowd made a railing collapse and dozens of people fell, crushing those at the bottom of the pile, according to a preliminary investigation.
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."
Eighteen-year-old Giulia Bucci said those trying to flee started pushing, adding: "It was a domino effect, some fell on me and I fell on others and my legs were squashed."
- 'You can't die like that!'
A pile of abandoned shoes testified to the chaos, with survivors and the bereaved weeping outside the nightclub as emergency vehicles tended to the injured, 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.
The crowd at the Lanterna Azzurra (Blue Lantern) club was attending a performance by Italian rapper Sfera Ebbasta, known as "the king of the trap" for his style of Hip Hop.
"I'm deeply saddened by what happened," Ebbasta said on Instagram.
"I don't want to pass judgement on who is responsible, but I would just like to ask everyone to stop and think how dangerous and stupid it can be to use pepper spray in a nightclub," he said, cancelling all his upcoming promotional events.
Pope Francis said during Saturday's Angelus prayers that he would pray for "the children and the mother who died last night... as well as for the injured."
The accident happened at around 1 am (0000 GMT).
"Usually there's no noise, one hardly hears the music just the laughter of the youths when they arrive or leave" the club, said Ines Patrignani, who lives in an adjoining house.
"But this morning, I was woken by the sounds of cries and sobbing," the 86-year-old said.
The injured were taken to hospital in the nearby town of Senigallia and the most serious on to Ancona.
Gabriele Santini said his 15-year-old granddaughter was still in shock.
"When I arrived she looked like a zombie. The moment she started speaking about what happened, she burst into tears," he said.
Italy's far-right Interior Minister Matteo Salvini, promised to "find those responsible for these six broken lives, those who through wickedness, stupidity or greed turned a festive evening into a tragedy."
In 2017, a woman died and 1,500 were injured in a crush during the outdoor broadcast in Turin of a Champions League football match final when rumours of a bombing sparked panic. (AFP)