Police went to the News Divine club in the working-class district of Nueva Atzacoalco in the early evening to check reports of drugs and alcohol being sold to minors.
Ortega told the Televisa network the club's owner announced to the crowd that the officers were there to arrest them, causing a stampede. He denied earlier media reports that officers threw tear gas inside the club, and also said no shots were fired.
At a news conference later Friday, Ortega said about 500 young people -- more than the club's capacity -- were there to celebrate the end of the school year and many more were waiting in line to get in.
Clubs in Mexico often host afternoon "tardeada" parties for clubbers as young as 16, though only patrons 18 and up are allowed to drink alcohol.
Ortega said the venue had a single entrance plus an emergency exit, which was obstructed by cases of beer.
"Many of the people concentrated at the club's emergency exit, but it was too small and it was there that people died of asphyxia," Ortega told Televisa.
Television images showed glittery high-heel shoes and T-shirts strewn over the dance floor. Bodies covered with white sheets lay by the club's entrance.
Ortega said the club, which charged a cover of US$3, was closed down last year due to several safety and other violations, but the owner won an injunction and reopened the place.
Police detained 39 people including the owner, he added.
Dozens of riot police were sent to control a crowd of sobbing relatives and friends that gathered at the scene.
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