Prosecutors have sought the death sentence for four men convicted of the gang rape and murder of a student on a New Delhi bus last December, saying the "diabolical" crime had shocked the country's conscience.

Judge Yogesh Khanna has heard arguments in a fast-track court as he considers how to punish the men who were found guilty this week of "cold-blooded murder". They will be sentenced on Friday.

Judge Khanna faces widespread calls from the public and leading politicians to hand down the death sentence, which can be given for "the rarest of rare" crimes but is seldom carried out in practice.

"The court should give the maximum sentence otherwise the message will go to society that deviance of this nature will be tolerated," special public prosecutor Dayan Krishnan told the packed court.

"The test is was the collective conscience shocked? There can be no better example than this case," he said, calling the crime "diabolical" in which "no element of sympathy" had been shown to the victim.

"The sentence which is appropriate is nothing short of death."

The 23-year-old victim, a physiotherapy student who cannot be named for legal reasons, died of internal injuries on December 29 after being lured on to the private bus following a cinema trip with a male companion.

After beating up the friend, the gang brutally assaulted the woman behind tinted windows for 45 minutes before flinging the bloodied, naked and barely conscious couple from the vehicle on a road.

Defence lawyers call for leniency

The four convicts - Akshay Thakur, Pawan Gupta, Vinay Sharma and Mukesh Singh - were led into the court by armed police and stood at the back of the court wearing t-shirts and displaying no outward emotion.

The mother of the victim was seated next to her husband as she listened intently to proceedings.

"The manner (in which) they committed this crime and destroyed a life, they should not get anything less than the death sentence," she told NDTV television after the men were convicted.

The lawyers acting for the men argued there was political pressure for an execution.

Vivek Sharma, a lawyer for Gupta who was 19 at the time of the crime, argued that life imprisonment should be the sentence for a crime committed on the "spur of the moment".

"The court must bear in mind that life imprisonment is the rule, and the death sentence is the exception," he said, adding there was a possibility of reforming Gupta because of his "tender age".

A fifth adult defendant, bus driver Ram Singh, was found hanging in his prison cell in March while awaiting trial.

The case brought simmering public anger over rape and harassment to the boil, sparking unprecedented protests, tougher new laws to tackle sex crime and a bout of introspection about India's treatment of women.

AFP