Delhi Police Commissioner Neeraj Kumar said four people had been arrested for what is amongst the most horrific rape incidents ever reported, putting the spotlight on the vulnerability of women in India's national capital.

Those arrested were bus driver Ram Singh, his brother Mukesh, fruit seller Pawan Gupta and Vinay Sharma, an instructor in the Delhi government-run gym in Siri Fort. Two others, Akshay Thakur and Raju, were absconding.

A Delhi Court later sent the main accused Ram Singh to five day's police custody.

According to police, the men were out on a joyride and had been drinking. But before perpetuating the brutal crime, the accused picked up a man and dumped him after robbing him of his valuables.

Doctors at Delhi's Safdarjung Hospital, who said they had never seen a rape victim who had been so grievously injured, said the young physiotherapist was still critical and continued to be on ventilator support. Her condition "slightly" deteriorated late evening.

"The patient is in critical condition and she is now trying to speak. We can say it is a grievous injury and her intestines are severely damaged," a doctor told IANS.

The brutal rape and torture occurred Sunday night when the girl and a male friend boarded a private bus with tinted glasses. It moved along the bustling south Delhi areas of Munirka, Vasant Vihar and Mahipalpur as the men raped and tortured the girl and beat her friend, using iron rods and more. The couple was stripped, robbed and thrown off the bus near Mahipalpur.

The male friend was also taken to Safdarjung but discharged after treatment.

As some bare, unspeakable details of the gang-rape began to emerge in the media, a frisson of insecurity went up spines across cities and towns but there was also palpable anger.

This found reflection not just in street demonstrations or candle light vigils - in the capital and in other cities - but also in parliament where MPs spoke in one voice to condemn the barbaric crime and demand speedy justice.

In a rare instance of both houses of parliament spending a considerable part of the entire day discussing a crime that was across most newspapers with banner headlines, there were emotional speeches from members with many calling for the death sentence for the rapists.

Amongst them was Leader of Opposition and Bharatiya Janata Party (BJP) leader Sushma Swaraj who demanded a statement from Home Minister Sushilkumar Shinde.

"Accused in such cases should be hanged," thundered Sushma Swaraj, adding that even if the 23-year-old survived she would be a "zinda laash", traumatised for life.

Speaker Meira Kumar termed the incident "shameful and horrifying" and urged the government to take immediate and stern steps in the matter.

"We share the concern of the house. Strong steps would be taken in the matter," Parliamentary Affairs Minister Kamal Nath said.

Congress leader Girija Vyas urged the house to pass a bill on sexual assault on women and said states should set up fast track courts to deliver justice in rape cases.

The anguish found echo in the Rajya Sabha too.

Samajwadi Party's Jaya Bachchan broke down in the house saying she wondered what would happen to the girl's family, while Trinamool Congress' Derek O'Brien said as the father of a teenaged girl he was scared.

Bachchan also said rape should be treated at par with attempt to murder.

"I stand here nervous and scared as the father of a 17-year-old daughter," O'Brien said.

"Rape is not just a women's issue. It's about men who stop behaving like human beings and start behaving like animals," he said.

As the members after members expressed outrage, Home Minister Sushilkumar Shinde said the Delhi gang-rape case will be tried by a fast-track court with a request for daily hearings.

Former women and child development minister and now Congress spokesperson Renuka Chowdhury hoped that the gang-rape would not "remain a mere statistic."

The National Human Rights Commission (NHRC) also issued notice to union Home Secretary R.K. Singh and Delhi Police Commissioner Neeraj Kumar, saying that the incident was a "grave violation of human rights".

Outside parliament, analysts tried to make sense of what had happened, the psychopath edge of the crime that gave it its bestiality.

Saying that this was a sociopathic crime, psychiatrist Sanjay Chugh told CNN-IBN: "The mindset of the perpetrators of gangrape is an amalgam of mob/herd mentality, disinhibition, utter disrespect for social norms and a certain knowledge that the so called protectors of law will either turn a blind eye or can be pressurised or simply bought with a certain sum of money."

Carrying slogans like "mere skirt se oonchi meri aawaz hai", Delhi's women - and men - who demonstrated this Tuesday asking for exemplary punishment like hanging or public hanging, agreed.