A 47-year-old woman has been sexually assaulted by a gang of men in Melbourne.

Police are investigating the attack which they say occurred near Chapel Street, South Yarra about 4.15am on Sunday.

The woman was found naked with her ankles bound and screamed to passers-by that she had been gang raped moments earlier, Fairfax Media reported.

Before police arrived, the woman told several people details of the assault and weapons used by her attackers.

Police were unable to confirm these details, but asked for anyone with information about the attack to contact them.