Police believe the teenager was involved in a physical altercation with another boy about 4pm at Pacific Werribee Shopping Centre in Hoppers Crossing.
The 18-year-old was taken to hospital with serious injuries.
9News understands shoppers were placed in lockdown while the incident unfolded.
One shopper, posting on social media, said it was “one of the most traumatic things” they’ve ever experienced.
“You hear about stories like this but never expect it to happen in the same setting as you. I still can’t believe this happened,” the woman said.
Police arrested a 16-year-old boy nearby.
He has since been interviewed and charged with recklessly causing serious injury, possessing a drug of dependence and possessing a controlled weapon.
The 16-year-old boy has been remanded to appear at a children’s court at a later date.
