"The fire spread in a minute and we were a mountain of people trying to escape," said survivor Ariel Monges, 25, who lost a friend and a cousin in the smoke and flames and was searching for another friend at a city hospital.
Four of the Republica Cromagnon club's six doors were tied shut with wire or padlocks, according to Interior Minister Anibal Fernandez.
Before the concert, the rock band playing at the club had warned the crowd not to shoot flares because of the fire hazard, Mayor Anibal Ibarra said.
But during the first song, an hour before midnight on Thursday, a group fired a flare, turning the venue into an inferno.
The mayor said the emergency exits appeared to be shut "so that people wouldn't enter without paying."
Hospital lists showed most of the victims were in their teens and 20s, but rescue workers discovered at least a dozen children in the club. Some fans had brought their babies, including an 11-month-old, to a makeshift nursery in the women's bathroom, witnesses said.
Most of the victims died from smoke inhalation.
"The problem was the thick black smoke. You couldn't see where the exits were. There were only two tiny doors and it was impossible for everybody to escape," one survivor told local television. The whereabouts of the club owner was unknown.
The fire, which authorities said was extinguished quickly, was the worst in the Americas since a supermarket fire in neighbouring Paraguay in August killed nearly 400 people.
Pope John Paul II sent his condolences to the victims' families in a letter to church authorities.