The label, the hottest on the Paris catwalk, said it had sent a written apology to the 150 women who were forced to wait in a cramped stairwell for three hours at the casting for its upcoming show on Sunday.
Some were shut inside in the dark while the casting directors went off for lunch, according to James Scully, a US casting director who took to Instagram to brand their treatment as "sadistic and cruel".
He claimed the directors involved were "serial abusers".
"Not only was this sadistic and cruel it was dangerous and left more than a few of the girls I spoke with traumatised."
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Scully also accused a major Paris fashion house, which he did not name, of trying to "sneak in" 15-year-old models for its show.
"It's inconceivable to me that people have no regard for human decency or the lives and feelings of these girls, especially when too, too many of these models are under the age of 18 and clearly not equipped to be here."
"Balenciaga took notice of issues with the model castings carried out on that day (February 26). The house reacted immediately," a statement added.
"Balenciaga condemns this incident and will continue to be deeply committed to ensure the most respectful working conditions for the models."
The fashion house -- whose show is on Sunday -- has become the hottest ticket on the Paris catwalk since the Georgian wunderkind Demna Gvasalia took the helm last year.
Scully, whose post garnered support from supermodels Joan Smalls, Helena Christensen and Carolyn Murphy, also accused the French label Lanvin of discrimination against black models.
But the brand's designer Bouchra Jarrar hit back strongly after showing her autumn-winter collection Wednesday.
"They insult the couture houses with their accusations of racism," she told AFP. "What are they talking about?" said the French designer, whose family comes from Morocco.
"Our dressing room is full of girls from every origin. In fact, there is not a single French person in it.