Const. Jean-Pierre Brabant of the Montreal police service said two groups of people coming out of bars on St-Laurent Boulevard got into an altercation around 3 a.m.
Two other men in their 20s were injured, but both are in stable condition.
"The third victim, a man who was in his 20s, was attacked and suffered injuries to his chest. He was transported in a critical condition to the hospital. Police officers were able to make two arrests. We're talking about two men aged in their 20s, and we're still looking for a third suspect in that event," Brabant said.
Police have set up a security perimeter around the scene, and witnesses are being interviewed.