A New Delhi court trying a teenager over a fatal gang-rape last December that shocked India deferred on Thursday announcing the first verdict in the case, lawyers said.

A juveniles' court has finished hearing the case of the youngest suspect, aged 17 at the time of the assault on a moving bus, and had been widely expected to announce a verdict on Thursday.

"The court has completed the hearing. The order has been deferred to 25th of July," public prosecutor Madhav Khurana told reporters who had massed outside the court.

The crime, which saw the 23-year-old student victim die of internal injuries inflicted during the attack, generated widespread anger about endemic sex crime in India to the boil.

Several weeks of sometimes violent protests pushed parliament to pass a new law toughening sentences for rapists, while a round of public soul-searching sought answers to the rising tide of violence against women.

The victim's family had called for him to be tried as an adult, alongside five men initially arrested over the assault on December 16 who face the death penalty.

The trial of the adult suspects -- one of whom died while in jail from a suspected suicide in March -- continues in a separate court but is expected to wrap up in the next few months.

The parents of the victim, whom AFP is also not naming in accordance with Indian law, were present inside the small juveniles' court on Thursday.

"We hope we get justice on July 25th," said the mother, who has previously called for all suspects to be hanged, before entering the court. Reporters were not allowed inside the courtroom.

The juvenile suspect, a runaway who reportedly left home aged 11, can be sent to a correctional facility for a maximum three-year term, which will take into account the time he has already spent in custody.

The teenager, the youngest of six children according to his mother, was employed to clean the bus allegedly used for the attack and often slept rough or inside the vehicle, reports say.

He has denied any involvement in the crime.

The maximum sentence of three years' detention is likely to cause further anger in India where the suspects, some of whom have been beaten up in jail, are public hate figures.

Amid pressure to put the juvenile on trial in an adult court, officials conducted an investigation to determine his age and concluded he was 17.

A government panel set up after the Delhi gang-rape to recommend changes to sex crime laws rejected calls to lower the age at which people can be tried as adults from 18 to 16.

The panel's report in January said India's justice system continued to "breed more criminals including juveniles in our prison and reformatory system by ghettoing them in juvenile homes".

The report, overseen by a retired Supreme Court judge, added that it was "completely dissatisfied with the operation of children's institutions."

Shahbaz Khan, from the "Haq: Centre for Child Rights", told AFP that there were "no proper care plans" for institutionalised children which undermined the intention of rehabilitating wrong-doers.

Ranjana Kumari, a women's rights activist from the Centre for Social Research, said police and the courts were still too slow to respond to the victims of sex crime.

"What we got was a good piece of legislation and an increase in the number of women with the confidence to report crimes against them. But so what? That's not good enough," she said.

Kumari said the teenager's likely punishment was too lenient, and he should have been tried as an adult.

"This is a very gruesome crime and he was almost an adult at the time it was committed," she said.