NEW DELHI: The father of the main suspect in the New Delhi gang-rape case said Tuesday he had found multiple injuries on his son's body as a post-mortem tried to establish the cause of death in his prison cell.

While prison authorities have said that 34-year-old Ram Singh committed suicide on Monday in his cell at Delhi's maximum-security Tihar Jail, his family and lawyer have both claimed that he was murdered.

Speaking to AFP at the government-run AIIMS hospital in central New Delhi after examining his son's body, Mange Lal Singh said he 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 before being ushered away by policemen.

The findings of the post-mortem, which was carried out on Tuesday morning by doctors at AIIMS, according to legal and hospital sources, will be presented later in the day at a court where Singh was on trial for murder, rape and kidnap.

The government has ordered an inquiry into the death, with Home Minister Sushilkumar Shinde calling it a "major lapse".

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 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.

Officials at Tihar Prison said Singh committed suicide before dawn on Monday morning, having strung up a noose in his cell that he attached to a grille on the ceiling.

His lawyer and newspapers have questioned how the heavy-drinking bus driver -- who had a damaged arm from a road accident -- had managed to hang himself without waking the other men in his cell or being seen by a guard.

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, luring on board passengers 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.