The juvenile court has finished hearing the case of the teenager, who was aged 17 at the time of the deadly assault which shocked India and sparked weeks of sometimes violent protests.

The verdict had been deferred from July 11 to July 25 and was again delayed on Thursday by principal magistrate Geetanjli Goel to August 5 because of a legal challenge.

"The reason is because there is a case pending in the Supreme Court," defence lawyer Rajesh Tiwari told reporters outside.

Politician Subramanian Swamy, the head of the Janata Party, has filed a petition arguing that suspects aged over 16 who are accused of serious offences should be tried in adult courts.

The crime, which saw the 23-year-old victim die of internal injuries inflicted during the savage attack on a moving bus, generated widespread anger about endemic sex crime in India.

Public outrage and protests pushed parliament to pass a new law toughening sentences for rapists, while a round of public soul-searching sought to explain the rising tide of violence against women.

The trial of the adult suspects -- one of whom died 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 juvenile, 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 four remaining adults could face the death penalty if convicted.

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

They have called for the killers of their daughter to be hanged and have criticised what they see as the leniency of the juvenile justice system, which seeks to reform rather than punish criminals under 18.

A sentence of three years' detention for the teenager would likely cause further public anger in India where the suspects, some of whom have been beaten up in jail, are hate figures.

About sixty journalists from the domestic and international media gathered outside the court on Thursday.