Police are scouring rooftops in Melbourne's CBD as they hunt for a man suspected of robbing an Elizabeth Street business.

A man was seen on the roof and in the roof cavity of the My Chemist store just after 1pm on Wednesday.

Officers searched roofs above the business and found a black duffel bag which they have now taken as evidence.

Police have also door-knocked the homes of city residents and used their balconies to see if they could spot the offender who is still on the run.

A police helicopter also searched the area.