33-year-old Singh hanged himself with the help of his clothes from the grills of his cell in Jail No 3 at around 5 AM, jail officials said.
"There is already an inquiry which has been ordered by Tihar Jail on suicide of the main accused. We are enquiring into the matter," RPN Singh, Minister of State for Home Affairs, said.
"Singh was not alone in the cell. Other inmates were present and a guard was also posted. But nobody came to know about it. Around 5 AM, he was found hanging," a senior jail official said.
Singh, prone to violent behaviour and mood swings, had suicidal tendencies and was under "suicide watch," the official said.
Singh, who was to be produced before court for its daily hearing, was rushed to the jail hospital where he was declared brought dead. His body will be taken to Deen Dayal Upadhyay Hospital for post mortem.
Forensic experts have visited the Jail premises to collect samples.
Mamata Sharma, National Women's Commission Chairperson, raised questions about the functioning of Tihar Jail, saying it was "shocking" that the administration could not protect the undertrial and demanded an inquiry.
Former Tihar director general Kiran Bedi said only an inquiry will tell under what special watch was he and what happened to that watch. "How did this man give this watch a slip? I think we need to wait for the inquiry," she said.
Sources said he was under depression for the past couple of days and last evening, he even did not have food.
Singh was arrested a day after the December 16 incident from near his home in RK Puram. His brother Mukesh is also an accused in the case along with four others.
The girl was raped in the bus by him and his five associates, including a minor, in south Delhi after brutally beating her and also her male friend. The girl died in a Singapore hospital on December 29.
Singh was the driver of the bus in which the girl was raped. His brother was driving the bus when the girl was sexually assaulted allegedly by them.
The six were on a "joy ride" on December 16 night when they crossed Munirka bus stop in south Delhi where the paramedic student and her male friend were waiting for a bus to Dwarka.
The victims boarded the bus and inside, they had an altercation with the gang after they passed lewd comments at the girl. When the victims objected, they were beaten up, the girl raped and later they were thrown out of the bus in semi nude condition.
The incident had evoked widespread outrage and demands for death penalty for the accused.
According to police, the six men were so confident they will not be caught that the assailants carried on with their routine chores the next day while one brought back the vehicle to his house.
Just before the rape, the gang looted a carpenter after giving him a lift in the same bus.
After the rape incident, Singh took the bus to Noida at the residence of the owner in the early hours of December 17 to avoid being caught.
But later Singh thought nothing would happen and brought the vehicle back to R K Puram in south Delhi where it was detected on secret information.
New Delhi: Ram Singh, the prime accused in the December 16 gang-rape of a 23-year-old girl here, allegedly committed suicide in a high-security cell in Tihar Jail early Monday morning.
First Published: Monday, March 11, 2013, 11:49