Indian prosecutors have demanded the death penalty for four men convicted of raping and murdering a 23-year-old trainee physiotherapist last December.

They said it was important to send a signal to the country that such crimes would not be tolerated.

"The sentence which is appropriate is nothing short of death," special public prosecutor Dayan Krishnan told the court in New Delhi.

"The common man will lose faith in the judiciary if the harshest punishment is not given."

Akshay Kumar Singh, Vinay Sharma, Pawan Gupta and Mukesh Singh stood at the back of the courtroom surrounded by policemen. They showed no emotion as Mr Krishnan spoke.

The parents of the victim, who cannot be identified for legal reasons, sat just a few metres away from the men.

Judge Yogesh Khanna, who found the four guilty of "cold-blooded" murder yesterday, heard arguments from the prosecution and defence on sentencing.

The minimum sentence the men could receive is life in prison. Judge Khanna will deliver his ruling on Friday.

"There is no element of sympathy in the way in which the hapless woman was tortured," Mr Krishnan said in laying out the prosecution's case for execution.

The woman's injuries were so severe that she died in hospital in Singapore two weeks after the 16 December attack on a bus.

All four of the men denied the charges.

Three of them said they were never on the bus, and another said he was driving the bus and knew nothing of the crime.

The prosecution said DNA evidence and bite marks on the woman's body placed the men at the scene.

Under Indian law the death penalty is reserved for the "rarest of rare" cases. Even when it is imposed, the authorities rarely carry out executions.

The case has resonated with thousands of urban Indians who took to the streets in fury after the attack.

The victim became a symbol of the daily dangers women face in a country where a rape is reported on average every 21 minutes and acid attacks and incidents of molestation are common.

In pleading for leniency for 19-year-old Pawan Gupta, lawyer Vivek Sharma said the court should take into account the roles of each of the men in determining their sentences.

Quoting from earlier judgments, he said judges should not be "bloodthirsty".

"You can't give capital punishment on demand," he said, appearing to refer to the public clamour for the men to be hanged.

Women's rights groups have welcomed the guilty verdict, but cautioned against giving the death sentence.

They said that research across the world has shown that capital punishment does not act as a deterrent and the case should not set a precedent for all rapes to be punished with hanging.

Others who oppose executing the men argue that putting them in prison for the rest of their lives is a harsher punishment than hanging them.

If Judge Khanna does give them the death penalty, India's high court will still have to confirm the sentences.

The four men are also expected to file appeals, so proceedings could still go on for months or even years.