A gang of six males sexually assaulted one woman and punched another in the face in a brutal overnight attack in central Melbourne.

Police say the same group may be responsible for a string of robberies across the city on Friday night and Saturday morning.

Sergeant Megan Stefanec said the two women had been walking down Queen Street just before 4am on Saturday.

"Two women were assaulted and had a speaker and tobacco stolen by a group of six male offenders," she said.

"One woman was sexually assaulted and the other woman was punched to the face before the offenders fled.