Youth offenders have attacked two people in separate incidents just hours apart in Melbourne.

An elderly man was seriously assaulted inside a chemist at Parkmore Shopping Centre on Cheltenham Rd at Keysborough about 4.55pm on Friday.

The 70-year-old man was assaulted by two teenage boys when he tried to stop them from leaving the store with suspected stolen items.

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He was taken with serious injuries to hospital.

Two hours later, a 16-year-old boy was waiting at a bus stop on Centre Dandenong Rd in Dingley Village when he was approached by what is believed to be the same two boys.

The boy was then stabbed in what appears to have been an unprovoked attack.

“He was also assaulted as he tried to get away,” Victoria Police said.

The boy was taken with serious injuries to hospital.

Local police and the dog squad quickly converged on the area and searched for the pair of teens, who fled from the scene.

A 19-year-old man was arrested on Saturday morning at an address in Bonbeach, near Frankston.

He was taken to hospital with non-life-threatening injuries and will be interviewed at a later stage.

“The second offender remains outstanding, and investigators will continue to make enquiries into his whereabouts,” Victoria Police said.

Anyone with any information or anyone who may have witnessed the incidents has urged to contact Crime Stoppers.