The trials of five suspects for the fatal gang-rape of an Indian student on a bus last December moved towards their conclusions on Thursday, with the first verdict set for August 31.

The Supreme Court cleared a juvenile court to hand down its verdict at the end of the month on a teenager charged over the brutal assault, which shocked India and sparked weeks of protests.

The juvenile court has four times delayed ruling on the under-age suspect because of a legal challenge, but the Supreme Court ruled that this should not hold up proceedings any longer.

Separately, the prosecution began its final arguments at a fast-track court in southern New Delhi where the trial of four adult defendants is underway.

Special public prosecutor Dayan Krishnan started his arguments by stating that the suspects had "pre-planned the conspiracy to loot, rape and murder their prey".

"What is so significant about this case is the extreme brutality of the injury. The extreme torture and the nature of the injury is such that it is clear they intended to murder their victims," he said.

"The brutality is grotesque to say the least," he stated in his submission before the judge.

Police allege the men repeatedly raped the 23-year-old woman who was also sexually assaulted with an iron rod. They then dumped her and a male friend along a highway naked and bleeding.

Her injuries were so severe that she died two weeks after the attack. The woman's friend -- who was with her on the bus -- was also severely beaten, police say.

Additional Sessions Judge Yogesh Khanna, who was been hearing the case since the trial began in February, will deliver his verdict once the arguments conclude. A defence lawyer said this could take "another few hearings".

"The prosecution will require at least another five days to present his final case. I will require at least 15 more sittings," defence counsel A. P. Singh told AFP outside the court.

Public outrage and protests over the attack 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 judgement on the juvenile was held up after politician Subramanian Swamy, an opposition leader, filed a petition in the Supreme Court arguing that suspects aged over 16 who are accused of serious offences should be tried in adult courts.

"The juvenile board can go ahead with its proceedings and pass orders accordingly," Chief Justice P. Sathasivam said.

Swamy said he was "completely satisfied" with the judgement of the Supreme Court. It agreed to hear his petition into a review of the juvenile law in general, but not specifically on the juvenile's case.

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

He was 17 at the time of the crime but is now 18.

One of the suspects originally arrested for the crime -- the driver of the bus on which the victim was brutally assaulted -- died while in jail from a suspected suicide in March.

The four adults could face the death penalty if convicted. All the accused have pleaded not guilty.

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