The problem began just before 5 a.m., when crews responded to reports of a water main break. They arrived to find water and smoke in the basement.
Residents said the building's lights were flickering on and off. Soon after, the building lost power entirely and the smoke alarms stopped working, residents told CBC News.
Residents were forced to exit their apartments and make their way out through dark, smoky corridors.
Toronto Fire said 105 people were evacuated from the building which has nine commecial floors on the bottom and eight residential floors above.
"The power started flickering on and off, probably around 5:20 in the morning," resident Jared Friedberg told CBC News. "We heard the fire alarm going in and out so we went to the stairs. We got to floor six and we could see and smell smoke. By the time we got to ground level, it was pretty hard to breathe."
The residents began to file out of their units and toward the stairs. No one was hurt, though some had trouble getting out due to the dark hallways and lack of elevator service.
"When I first got to the stairwell I was disoriented. I didn't know which way was up or which way was down, so I started feeling my way down to the bottom of the building," said building resident and CBC business commentator Michael Hlinka on Metro Morning.
"When I hit the bottom of the building the smoke was, at that point, quite thick, and I couldn't see. There was no exit sign that showed me where the exit door was so I started to make my way back up, hoping to find an alternative exit. Fortunately a couple of neighbours came down with flashlights and we all made our way to safety."
Outside, some residents boarded TTC buses sent to the scene to keep residents warm.
"I can't say that I was terrified, but what I can say is that you could really see how something like that could lead to a stampede," said Hlinka. "What came to mind was what happened in Brazil for some reason. You could really see how something like that could make people panic."
Animals removed from clinic
A number of pets were removed from an animal clinic located on the building's main floor. Many were staying at the clinic overnight to recover from surgeries.
CBC overnight reporter Tony Smyth captured images of the animals being removed from the building. His photos proved popular with Twitter users on Wednesday. One showed a small dog breathing oxygen from a tank. There were no animal injuries reported.
"Just like the residents here, all the animals are okay," reported CBC's Trevor Dunn.
Fire officials said damage to the hydro vault is extensive. Toronto Hydro is handling the repairs to the vault. Toronto Fire division commander Andrew Kostiuk said the same vault also houses the building's emergency power generator which is why the emergency lighting system and the fire alarm failed to function.
Fire officials also said it could be days before residents are allowed to return to the building.
Yonge Street near the scene remained closed to traffic in both directions.
A Twitter message sent by police just before 11 a.m. said Yonge Street would be close to traffic for "several hours."