An Indian teenager was sentenced to three years in juvenile detention on Saturday for the December gang rape of a trainee physiotherapist, the first verdict in a case that sparked debate over whether India is too soft on young offenders.

Police say the 18-year-old and five adult men lured the 23-year-old woman and her male friend onto a New Delhi bus where they repeatedly raped her and beat them both with a metal bar before dumping them on to a road.

The woman died in a Singapore hospital two weeks after the December 16 attack. The case turned a global spotlight on the treatment of women in India, where police say a rape is reported every 20 minutes.

"The juvenile has been found guilty under rape and murder charges, and accordingly sentenced to three years of jail," Rajesh Tiwari, a lawyer for the juvenile, told reporters.

The lawyer said the juvenile would be sent to a reform home to serve the term, taking into account the months he has already spent in detention since his arrest.

Four of the teenager's co-accused are still on trial and face the death penalty if convicted. Closing arguments began on August 22 and verdicts are expected within the next fortnight. A fifth accused, the alleged ring-leader, killed himself in his jail cell in March.

The teenager, who may not be named, was tried as a juvenile as he was 17 at the time of the attack. The maximum penalty that could be imposed by the Juvenile Justice Board was three years.

In January, authorities ruled he was 17, citing school records, which shocked the victim's family and others clamoring for him to face the death penalty.

"You may as well set the juvenile free, if the sentence is only three years for heinous offences like rape and murder," said the victim's mother who was in tears after the verdict.

The mother also said she would appeal against the verdict in a higher court.

"I am not happy with this judgment. At least in this case, the juvenile should have been sentenced to life," the victim's brother told Reuters.

In response to the public outcry after the rape, the government fast-tracked tougher laws against sex crimes, but it resisted calls to change the juvenile law and return the adult age to 16 from 18.

India's Supreme Court is currently hearing a petition filed by Subramanian Swamy, an opposition politician and lawyer, that calls for the law to be reinterpreted rather than changed.

Swamy wants a minor offender's "emotional, intellectual and mental maturity" to be assessed when deciding whether to try them as a juvenile, rather than basing the decision on age alone.

"I felt that, with the kind of rape that took place, if (the juvenile suspect) got off lightly it would send a bad signal to society," Swamy told Reuters. He plans to launch an appeal against the verdict reached on Saturday if the Supreme Court rules in favor of his petition later this year.

The teenager pleaded not guilty to all 13 charges including rape, murder and robbery. His trial was held behind closed doors to protect his identity and media were barred from reporting on any details of the proceedings.

During his trial, he has been held at a juvenile detention facility for violent young offenders in Delhi and kept away from other inmates for his safety.

The youth left home when he was 11 and got work in a roadside eatery, his mother told Reuters in January. In recent years he lived as a semi-vagrant, washing buses and collecting fares, according to a police report.

After leaving home, he never returned and his mother said she thought he was dead until police arrested him in connection with the gang rape.

Some 33,000 crimes were committed by juveniles in India last year, the highest number in a decade, but there has not been a large spike, according to Home Ministry figures. Juveniles commit a tiny proportion of total crimes in India and far less than other nations such as the United States.-Reuters