A 74-year-old woman has been threatened with a knife during one of two terrifying carjackings in Melbourne’s south-east.

The attack happened in the car park of a shopping centre in Beach Street, Frankston at about 4.30pm on Tuesday.

The woman had just finished loading groceries into her vehicle when a man armed with a knife got into the passenger seat and demanded she drive.

The armed carjacker told the woman get out at a service station in Frankston North.

Thankfully she was not injured.

The man later crashed the vehicle before stealing a Toyota Camry at knifepoint in Seaford.

Police pursued the vehicle, deploying stop sticks and arresting a 30-year-old Carrum Downs man with the help of the dog squad.

The man is in hospital under police guard this morning.

Press PLAY below for more details from police