Police say the guard — one of two operating the armoured truck owned by the company G4S — was ambushed just after 1 a.m. ET as he got out of the truck to make a delivery to the TD bank located at 1677 Avenue Rd., just north of Lawrence Avenue West, in the city's north end.
Witnesses told CBC News that multiple shots were fired.
Paramedics say the victim, a man in his 40s, was rushed to Sunnybrook Hospital with life-threatening injuries.
Toronto police say they have three suspects into custody and have recovered a firearm.
The suspects were arrested in the nearby Lawrence Heights area.
Police say there was no cash taken in the robbery attempt. There is no word yet on charges.