The incident came to light after the victim suffered severe pain in her private parts and was admitted to a hospital in Begumpura in her hometown of Aurangabad for treatment.
The doctors suspected it could be a case of rape and informed the police.
The girl, who had kept the matter secret from her family, finally revealed the details of the incident.
It was on July 7, when the victim had come to Mumbai and her four male friends, whose identities are not yet clear, insisted on celebrating her birthday at the home of one of them.
That evening, after cutting the birthday cake, the four friends allegedly overpowered and gang-raped her, and the victim, in deep shock and trauma, quietly returned home the next day without informing the Mumbai police or her family.
However, it was only in the third week of July when she complained of severe pain in her private parts that she was taken to the hospital where the matter ultimately came to light on July 30.
"We have immediately registered a 'Zero' first information report (FIR) on July 30 and dispatched the entire case file to Chunabhatti police station in Mumbai which will now investigate the matter," an official of Begumpura Police Station told .
The victim's condition has been described as "very serious" by the police and she is suspected to have suffered multiple internal injuries due to the gang-rape and is under continuous observation.
A police team has left for Aurangabad to take the victim's statement.
( With inputs from IANS )