The police were called at about 2.50pm local time (7.50pm BST) after reports of gunshots.
Toronto Police said on their official Twitter account: “Sounds of gunshots Yorkdale.
“Officers have confirmed a discharge in the area.
“Yorkdale Shopping Centre is in lockdown. Nobody being let in.
“People being assisted to leave. Officers canvassing for witnesses and video from witnesses, mall and stores.
“No injuries. East side of mall is location of incident.”
A worker in the mall, Carolina Toca, said she heard three or four shots fired.
She said: “Everyone started to run like crazy.”
Ms Toca ran to a store nearby where she said she was locked in with about 15 other people.
Toronto police said one person was taken from the centre by paramedic services.
But this is not a result of the shooting.
There has been no reports of injuries so far.
Police said they are looking for multiple suspects.
They described one as a male in his 20s wearing a black jacket with a hoodie and blue jeans.
A second is described as a skinny male in his 20s who is around five-foot-nine and carrying a black handgun.
Any witnesses are asked to call police at 416-808-3200.
Social media posts show mall workers and shoppers under lockdown as alarms ring out through the facility.
Toronto Transit Commission said subway trains are not stopping at Yorkdale due to “police activity”.
The mall is Canada’s fifth-biggest and opened in 1964.
Yorkdale Shopping Centre said the mall would be closed for the rest of the day as police investigate.
This is a developing story, more to follow…