A post-mortem on a man suspected of leading the gang-rape of a student on a New Delhi bus last December has concluded he died by hanging, a person close to the case told AFP on Tuesday.

Ram Singh, one of six men on trial over the assault which appalled India and sparked mass protests, was found dead in his cell in high-security Tihar Jail on Monday morning where he had been held since his arrest after the crime.

Amid allegations from his family that he had been killed, the government ordered an investigation and the post-mortem was seen as crucial in determining whether it was a suicide as alleged by jail officials.

"All the investigations conducted by us show it's death due to hanging," said the source, who was speaking to AFP on condition of anonymity, adding that three doctors at the state-run AIIMS hospital had conducted the autopsy.

The report is set to be submitted in court on Wednesday, he said.

Singh's father Mange Lal told AFP on Tuesday that he had examined his son's body and had found "multiple injuries" including marks on his chest, face and an injured eye.

"My son was murdered by the three inmates in his cell," he shouted at the hospital before being ushered away by policemen, an allegation repeated by Singh's lawyer V.K. Anand.

The source connected to the case said the post-mortem showed only the cause of death and did not reveal whether he had been murdered or not, which was a matter for the police to investigate.

Singh's family said they would take his body back to a village in Rajasthan for the last rites to be held.

While the post-mortem will likely quell some of the speculation about the death, prison authorities will still face serious questions about how the bus driver could have hanged himself from a grille on the ceiling.

He is said to have made a noose out of his clothes and sheets in the cell and succeeded with his suicide bid without waking the other men in his cell or attracting attention from guards who are meant to monitor suspects.

Singh was charged along with four other men and a 17-year-old over the brutal December 16 attack on a physiotherapy student who was lured on to a bus and repeatedly raped.

Left with horrific internal injuries, the 23-year-old died 13 days after the assault, leading to an outpouring of anger over endemic crime against women in India and a review of rape laws.

Singh was accused by police of being the ringleader in the gang-rape and his parents said Monday that he was ready to accept his punishment having confessed privately to taking part.

On the night of the attack, the suspects allegedly gathered at Singh's home for dinner and drinks before taking the bus for a joyride, fooling travellers who mistook the vehicle for genuine public transport.

Singh, a migrant from Rajasthan state, was the first of the accused to be arrested after police pulled over the private bus, which was normally used to ferry schoolchildren.

All five adult suspects including Singh pleaded not guilty to the charges.