A Melbourne mother and her young son have gone missing.

Ly Ny, 29, and her eight-year-old son Leon, left their Cranbourne home in the city's southeast on Saturday morning and have not been seen since.

Ms Ny's mobile phone, however, was found at a shopping centre later on Saturday and staff were able to call her family.

The disappearance comes as police launch the 25th National Missing Persons Week.

Police say more than 8500 people have gone missing in Victoria in the last 12 months, more than half of whom were under 17.