Jeannette Zacarias Zapata, who is 18, was taking part GYM Gala International Boxing event at the IGA stadium in Jarry Park.
She was knocked out by Quebec fighter Marie-Pier Houle at the end of the fourth round of a six-round fight.
The event's promoter, Yvon Michel, said Zapata's condition has remained stable but that doctors say the next 48 hours are critical to her survival.
Accidents like Zacarias's are very, very rare and we want to make sure to find ways so it doesn't happen again, Michel said in an interview Monday morning.
Boxer had medical tests to get permission to fight
Three years ago, Montreal boxer and world champion Adonis Stevenson spent three weeks in a coma (new window) after a knockout resulted in a serious brain injury.
Zapata had undergone neurological exams to get permission to participate in the Montreal fight, Michel said, because she was knocked out in a match last May in Mexico.
During the fight, Houle struck Zapata with a left uppercut and then delivered a final right hook, jolting her opponent's mouth guard.
Unable to recover after the bell rang, Zapata went into convulsions. Her trainer, on-site paramedics and a doctor assigned to the event rushed to help her.
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Zapata spent minutes unconscious in ring
It was not known if Zapata was conscious as she was carried out of the stadium on a stretcher.
In her first match in 10 months, Houle showed uncharacteristic punching power to cut the fight short.
Despite her victory, she said she was more concerned for her opponent, who spent minutes lying unconscious in the ring.
From an athletic standpoint, Houle said she was unsatisfied with her performance in the first two rounds.
I was standing too far away. My shots did not come out, she said.
But in the last 30 seconds, I opened the machine and you saw what happened, said Houle, visibly upset after learning about Zapata's condition.
CBC News with files from Elias Abboud, Chloe Ranaldi, Radio-Canada's Jean-François Chabot and Canadian Press