According to police, three people were taken to hospital for treatment. The extent of their injuries is not immediatley known. A fourth injured person was treated at the scene.
Police said their preliminary investigation suggests it was a random attack, adding the victims did not know the assailant.
"The suspect randomly attacked people in close proximity," police said in a statement Tuesday night. "There is no indication as to the suspect's motive at this point."
Two police officers happened to be in the mall at the time as part of their routine patrol and were able to get to the scene right away.
They ordered the youth to drop the knife. He threw the weapon at them and was then subdued and taken into custody without any further incident.
Several police officers are still on the scene and a portion of the mall, outside a coffee shop, was cordoned off. The mall was later closed.
The ages of the victims or the person in custody were not immediately known, but all are males.
One witness took a photograph of a young person, clearly injured, being tended to by emergency crews.
CBC News has a reporter on the scene, gathering additional information.