A man with a shopping trolley has been praised for trying to subdue a knife-wielding attacker who had just stabbed three people in Melbourne.

People are at the scene of a major operation in Bourke St in the CBD after a vehicle crashed and exploded about 4.20pm.

The driver then got out and began stabbing pedestrians in a terrifying frenzy, injuring two and killing one, police say.

Video shows the dramatic moment the attacker lunges at two responding police officers with the knife.

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Simultaneously, one heroic eyewitness rushes at the attacker with a shopping trolley, briefly slowing him down and falling in the process.

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.