Suffolk Police said the victim, in her late 20s, was attacked in Ipswich by two men sometime between 03:00 and 03:45 GMT on Saturday.
She was walking along Rope Walk and crossing Grimwade Street when she was grabbed from behind by the men who forced her into a nearby car park.
She suffered neck and face injuries.
Trained specialist officers and police staff have been working with the victim whilst extensive inquiries are ongoing in the local area.
Det Supt Eamonn Bridger said, "This is a despicable, sustained sexual attack by two unknown individuals on an innocent young woman, who has been left traumatised and with significant injuries as a result of the actions of the men involved".
The force is appealing for witnesses, including anyone with dash cam footage who was driving in the area at the time of the offence.
A woman has been left unconscious and seriously injured after what police have described as a "despicable, sustained sexual assault".