Meanwhile another bystander closes in while holding a chair from a nearby cafe.
The first man was not deterred, however. He sprung back into action and charged back towards the attacker, shoving the trolley back in his direction as police opened fire.
"Total respect to the shopping trolley hero trying to take down a mad knife man with nothing but a metal basket on not very good wheels," Paul Cochrane tweeted.
"Incredible work by police in Melbourne but also bystanders — one using a trolley to try subdue the man," Cyndi Lavrencic said.
"Cops trying to take down man armed with knife. Passer-by seems to be trying to take him out with a shopping trolley!" Dan Nolan said.
The attacker is in hospital in a critical condition and investigators have not released any information about his identity.
The bomb squad is on the scene.