A man has had his brand new Mercedes stolen at gunpoint in a terrifying carjacking outside Chadstone Shopping Centre.

Police say the carjacking took place in the carpark of the shopping centre, in Melbourne's south-east, just after midnight on Tuesday.

A man has been carjacked outside Chadstone Shopping Centre.

A 39-year-old Oakleigh man pulled up outside the entrance to a supermarket on the Dandenong Road side of the shopping centre when a dark-coloured car stopped behind him.

An armed man got out of the car and demanded the victim's car keys at gunpoint, police said.