NEW DELHI - India's top court on Friday upheld the death sentences of four men convicted for the fatal 2012 gang rape of a Delhi student, an attack that triggered global outrage and national soul-searching about the treatment of women.

The Supreme Court said the 23-year-old woman had suffered a "devastating hour of darkness" as it rejected an appeal against the death penalty, which was handed down in 2013. "If at all a case warrants the death sentence it is this case," said Judge R Bhanumati. Jyoti Singh, a physiotherapy student, was raped and left for dead by a gang of five men and a teenager after she boarded a private bus while going home from the cinema with a male friend. She died of grievous internal injuries 13 days later. The brutality of the attack, during which she was penetrated with an iron bar, triggered what the judges on Friday described as a "tsunami of shock" in the Indian capital.

Thousands took to the streets to demand the government tackle the threat to women and urge the harshest punishment for Singh's attackers.

Four men were convicted in September 2013 for murder, gang rape, theft, conspiracy and "unnatural acts" after a seven-month trial in a fast-track court.

At their sentencing, Judge Yogesh Khanna said the case fell into the "rarest of rare category" which justifies capital punishment in India.

A fifth man, the suspected ringleader, was found dead in jail in a suspected suicide, while the 17-year-old was sentenced to three years in a detention centre and has since been released.

"This is a historic decision and will set an example for the courts in dealing with criminals," Singh's father Badrinath told AFP outside the court.

He said he and his wife would urge the president, who has the power to commute a death sentence, to reject any mercy petitions and ensure they were hanged "as soon as possible to conclude the process of justice".

Lawyers for the four said they would file a review petition - the next stage in the appeals process.

"We are sure the court will change its decision once we appeal and justice will be delivered to the poor," said A P Singh, which represents three of the four, outside the court.

The defence had argued that the men should be shown leniency in view of their young age and poor backgrounds.

Media reports said a round of applause went up in court as the judgement was read out on Friday at the end of an appeal hearing that began last year.

Women's rights activist Ranjana Kumari called it a "historic judgement".

"This is a historic message to all the people, the criminal mindset who wrong women, who inflict violence on women, to know that if you do something like this you will be also paying for it by the severest punishment that exists in our laws of the land," she said on Indian news channel NDTV.

The accused - low-paid migrants to New Delhi - brutally assaulted Singh behind tinted windows for 45 minutes, penetrating her with an iron bar.

Her injuries were so severe that she died nearly a fortnight later in a Singapore hospital. She had only briefly regained consciousness, telling family and friends of her desire to see her attackers burn to death.

The case prompted the government to amend the law to introduce tougher punishments for rapists, including the death penalty for repeat offenders.

Nonetheless, levels of sexual assault remain extremely high in the Indian capital, which registered 2,199 rape cases in 2015 - an average of six a day.

Activists say the true number is likely much higher, with many attacks going unreported because of the social stigma attached to sexual crimes.

India has more than four hundred people on death row, but has carried out only four executions in the last decade following an eight-year unofficial moratorium.

In 1983, the Supreme Court said that death sentences - carried out by hanging - should only be handed down by judges in lower courts if the crime was the "rarest of the rare".

But Indian public opinion remains strongly in support of capital punishment with celebrations held in November 2012 when a gunman from the 2008 Mumbai attacks was put to death in the first execution in eight years.