A gold Volkswagen Passat was seen driving erratically moments before colliding with a blue Alfa Romeo along the busy Lygon Street precinct in Carlton about midday on Saturday.
The Alfa Romeo spun out of control and hit a male pedestrian crossing at the intersection with Elgin Street.
Detective Sergeant Stephen Hill said bystanders rushed to help and spoke to the driver of the Volkswagen, keeping him at the scene until law enforcement arrived.
Footage from the incident shows several people pinning a man to the ground until police arrived.
"I'm in awe of any member of the public that stops at scenes like this," Det Sgt Hill told reporters.
"It's very confronting and very traumatic."
A 16-year-old girl from North Brunswick who was in the back seat of the Alfa Romeo, suffered critical injuries while her 53-year-old mother was seriously injured. Her father, 61, was also hurt.
No identifying details have been released about the pedestrian however Det Sgt Hill said he was up and awake in hospital.
"Quite a lucky man under the circumstances," Det Sgt Hill said.
An Essendon man, 40, is under police guard in hospital and assisting investigators.
Det Sgt Hill said it was too early to know what caused the crash but the Volkswagen driver will be tested for drugs and alcohol.
A witness, who did not want to be named, told AAP he heard a "massive explosion" and was shocked to see a large traffic pole knocked to the ground.
Several blocks around the popular Carlton precinct were blocked off on Saturday afternoon and into the evening as authorities collected evidence and cleared the debris.
Anyone who witnessed the lead up to the crash or drivers with dashcam footage have been asked to come forward.
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