Nirbhaya became a symbol for widespread campaigns in India on women's rights and safety (File)

Supreme Court on Monday upheld death sentences for three men over the infamous gang-rape and murder of a woman in New Delhi in 2012 that sparked national protests and led to tough new penalties for sexual violence.

The horrific violence meted out to 23-year-old on a bus in the national capital sparked angry protests and shone unprecedented attention on the scourge of sexual crimes against women in the country.

The case spurred a major overhaul of laws governing such crimes, faster prosecutions in courts and harsher punishments for perpetrators.

Here are five questions and answers about the watershed case.

Who was the victim?

Nirbhaya's father moved the family from their small farming village in rural Uttar Pradesh to the bustling capital in search of work.

It was hoped Nirbhaya would become the first professional in the family, and all energies were channelled into her studies at a private physiotherapy college.

Her father earned just $200 per month as an airport baggage handler but sold ancestral land to help pay for his daughter's tuition.

To supplement her family's meagre income, Nirbhaya worked nights at an outsourcing firm and gave private lessons to school children.

What happened on December 16, 2012?

Nirbhaya was returning from the cinema after watching 'Life of Pi' with a friend when they were offered a ride in a private bus.

Six men, including a juvenile, beat Nirbhaya's friend unconscious before raping and torturing her with an iron bar as the bus drove loops through the city.

She was dumped on the streets 45 minutes later with horrific internal injuries, and died 13 days later in a Singapore hospital.

Why is this case extraordinary?

The savagery of the attack proved a tipping point in a city where an average of six women is raped every day.

Thousands of protesters took to the streets of Delhi demanding swifter justice for victims and tougher laws to punish perpetrators.

It also led to much soul-searching about the treatment of women in India, which suffers from extremely high levels of sexual assault and where discrimination against women and girls is rife.

As Nirbhaya struggled to survive long enough to help police identify her attackers, she became a symbol of India's long-overdue campaign to end sexual violence against women.

What happened to the attackers?

The accused were low-paid migrants to New Delhi, among them a bus cleaner, a gym assistant, a fruit seller and a school dropout.

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

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.

Sentencing the four in 2013, Judge Yogesh Khanna said the case fell into the "rarest of rare category" which justifies capital punishment.

Just three were involved in the appeal rejected on Monday.

What changes came of it?

In the aftermath of the crime, a panel headed by a former chief justice was tasked with reviewing laws on sexual violence.

The panel recommended a minimum sentence of 20 years for gang-rape, with the possibility of life imprisonment.

It also called for tougher punishments for a range of sexual crimes common across India, including voyeurism, stalking and acid attacks.

The government responded by introducing tougher punishments for rapists, including the death penalty for repeat offenders.