"I heard screaming," Sherene Bromfield, a resident of the building, told CTV News Toronto. "I ran on my balcony and I looked over and I could see her trying to revive her daughter.
"They said they had a pulse, a low pulse, and she just started crying."
The child's siblings, ages 5 and 7, saw the incident, the television network reported.
In a statement, officials with the Toronto Community Housing Corporation (TCHC) said it was “deeply saddened” by the death of the child.
“Our thoughts are with the family at this difficult time, and we offer them our sincere condolences. TCHC staff are onsite at the building today to assist tenants who require support,” the statement said.
Toronto Mayor John Tory said the toddler's death was a "terrible tragedy."
"My thoughts are with the family involved and the people in that building. I'm sure it is traumatizing for them," CBC reported.
The investigation is ongoing, police said.