India executed four men on Friday for the gang-rape and murder of a woman on a Delhi bus in 2012 that sparked huge nationwide protests and international revulsion.

The four were hanged before dawn at Tihar Jail in the Indian capital, the head of the prison Sandeep Goel told AFP, in India's first execution since 2015. "All four convicts (were) hanged at 5:30am," he said.

The brutal attack on Jyoti Singh sparked weeks of demonstrations and shone a spotlight on the alarming rates of sexual violence and the plight of women in India where around 95 rapes are reported daily.

The execution sparked small celebrations outside the prison early on Friday.

"We are satisfied that finally my daughter got justice after seven years," the victim's mother Asha Devi told reporters outside the jail. "The beasts have been hanged."

"Today, all Indian women received justice," Delhi resident Meena Sharma told AFP, clutching an Indian flag.

"I came here around 3:00am in the morning. I waited here as today is a great day for us."

Singh, 23, was returning home from the cinema with a male friend on the evening of December 16, 2012 when they boarded a Delhi bus, thinking it would take them home.

Five men and a 17-year-old boy had other, darker ideas. They knocked the friend unconscious and dragged Singh to the back of the bus and raped and tortured her with a metal rod.

The physiotherapy student and the friend were then dumped on the road. Singh died 13 days later in a Singapore hospital from massive internal injuries.

"A decent girl won't roam about at 9pm," one of the perpetrators later told a BBC documentary that was banned in India.

Indian prime minister Narendra Modi in a post on Twitter said that "justice has prevailed".

'Bursting dam'

Nearly 34,000 rapes were reported in India in 2018, according to official data. This is considered the tip of the iceberg, with many more victims too scared to come forward.

But Singh's ordeal, and the fact that she was part of a generation of young women trying to break out of a still very traditional society, struck a chord.

"It was like the bursting of a dam," said Kavita Krishnan, a women's activist who took part in the huge protests.

"It was not restricted to seeking revenge. Women said they do not want to trade their freedom for safety ... There was a social awakening of society," she told AFP before the hangings.

The uproar over the case led to tougher punishments for rapists including the death penalty for repeat rape offenders.

Singh, nicknamed "Nirbhaya" ("fearless"), survived long enough to identify her attackers and all six were arrested. Four were convicted in 2013.

A fifth, the suspected ringleader, was found dead in jail in a suspected suicide, while the 17-year-old spent three years in a juvenile detention centre.

India "has given a strong message to rapists that if you commit this crime you will be hanged," tweeted Swati Maliwal, chairperson of the Delhi Commission for Women.

But for Krishnan, the executions mask the continued failure to provide justice and improve safety for women in the world's biggest democracy.

Almost 150,000 rape cases are awaiting trial in India's dysfunctional criminal justice system.

The government is "trying to fix the public gaze on the gallows to divert attention away from what it has failed to do," Krishnan said.