Just after midnight, police were called to a wholesale business near the intersection of Henri-Bourassa Boulevard and Drapeau Avenue after receiving reports of a fight.
When they arrived, they found as many as 20 young people fighting in the street. Police said they also found an illegal party taking place in the basement of the business.
They called the Montreal fire department for advice before advancing in to break up the party.
As they were waiting, another fight broke out among the party-goers. The 36-year-old officer was knocked down and struck her head either on a post or concrete stairs, said Const. Simon Delorme.
"She suffered major injuries [to] her head," he said. "She's at the hospital right now, but there's no threat to her life."
The officer has 15 years experience, he said.
Police arrested 14 people, including four youths. They face charges ranging from assaulting a police officer, to intimidation to breach of conditions.