Along with a male friend, the student was hit with an iron rod before being thrown onto the road. A passer -by who saw them phoned the police, which then moved the young couple to hospital a little after 11 pm.
Police sources said that the attackers were most likely drunk and hit the victim and her friend repeatedly with brute force for nearly 90 minutes.
The 23-year-old victim is struggling for her life. She is on a ventilator after multiple surgeries, doctors said. Her friend is being treated for head injuries.
Security cameras on the main highway NH8 allegedly furnished footage which is helping the police.
The police said that after boarding the bus from Munirka in South Delhi, the couple got into a fight with a group of men over the route the bus driver was meant to take.
The victim and her friend, who works with a software firm, watched a movie at Delhi's Select City Walk mall in Saket last night; they then boarded a chartered bus for Dwarka.
About 10 minutes later, a few people on the bus began harassing the woman, said a relative of the boy who was accompanying her. "They took out a rod and beat him badly and then dragged her to the bus cabin where they raped her," the relative shared.