Zoo workers were surprised Tuesday night when Aurora gave birth to two male cubs and a female, as it was at least six weeks ahead of schedule, the Toronto Star reported.
An unidentified source told the Star the mother bear ate one of the cubs and badly clawed a second before workers could remove the surviving cubs.
The two little bears are under 24-hour care in the veterinary intensive care unit, Postmedia News reported.
"They will continue to be hand-raised and we are hopeful that they will both get stronger but they are far from being out of the woods at this time," the zoo said in a statement.
Aurora and her sister Nikita have been in the Toronto Zoo since 2001, when provincial wildlife officials found them orphaned by a hunter near Hudson Bay.
Zoo officials said the birth is likely the earliest ever recorded among captive polar bears.
Various countries and agencies rank the species as vulnerable to threatened or endangered.