MPs from all parties have expressed their horror at the incident which has caused outrage in the country.

Police said they have arrested four people, including the bus driver, and are looking for two more people.

The student and her male friend were beaten, stripped and thrown out of the bus on Sunday evening.

They have been admitted to hospital, where the woman remains in a critical condition.

"She continues to be serious. She has responded to us once in the last 12 hours and that is a good sign, but we cannot rule her out of danger. She remains on ventilator," Safdarjung Hospital medical superintendent BD Athani told the BBC Hindi service.

Delhi's rape figures are far higher than other Indian cities of comparable size, correspondents say.

According to official statistics, 572 rapes were reported in Delhi last year. Police say they have registered 635 rape cases this year.

On Tuesday, angry MPs in both houses set aside their regular business to discuss the gruesome rape case and demanded strict punishment for those who carried out the attack.

Senior leader of the main opposition Bharatiya Janata Party Sushma Swaraj called for the death penalty for rapists.

There has been outrage in India over the incident

"What is the government doing to curb rape cases in the capital?" she asked. "The rapists should be hanged, we need tougher laws to stop rapes," she said in the Lok Sabha, the lower house of parliament.

"I speak for the house when I ask the government to take the strongest measures to curb such incidents," Speaker Meira Kumar said.

In the Rajya Sabha, the upper house, MPs also demanded strict action against rapists.

"How is it that these rapists think that they'll get away with it?" asked an angry Renuka Chowdhury of the Congress party.

The leader of the Bahujan Samaj Party, Ms Mayawati, said: "We need to ensure proper investigation, and strict punishment. It's not enough to just arrest them, but action should be so strict that no one should dare to act in such a manner again."

Home Minister Sushil Kumar Shinde vowed to make the city safer for its women.

He said a new committee, headed by the home secretary, "will evaluate what needs to change, based on suggestions made in parliament by the MPs".

Mr Shinde said that routes used often by women at night would be identified and patrolled at night by police.

The minister said he would propose to "have this case tried by a fast-track court so that this case does not linger".

Delhi police chief Neeraj Kumar told a press conference on Tuesday that the woman and her friend had boarded the bus in the Munirka area of south Delhi.

They got on the bus and bought tickets for 10 rupees (18 cents; 11 pence) each, he said.

"The victim's male friend fought the attackers bravely, he tried to save his friend, but he was badly beaten up," Mr Kumar said.

"The woman tried to protect him, she was also beaten up and then taken to the back of the bus where she was gang raped. The two were then thrown out of the bus," he said.