A 23-year-old Indian student has been allegedly gang-raped inside a bus in the capital, Delhi, police say.
The woman was travelling with a male friend on Sunday night when they were attacked. They were brutally beaten, stripped and thrown out of the vehicle.
Both have been admitted to hospital. The condition of the woman is reported to be "critical".
Delhi is often called the "rape capital of India" - police recorded more than 550 cases in the city last year.
Correspondents say Delhi can be a very difficult city for women, with sexual harassment commonplace and rapes and abduction all too frequent. The capital's rape figures are higher than for other Indian cities of comparable size, they say.
Police have registered a case and Delhi Chief Minister Sheila Dikshit has promised "strict action" against those responsible.
The woman and her friend had boarded the bus from Munirka area and were on their way to Dwarka in south-west Delhi.
They were returning after watching a film in a mall in south Delhi, police said.
The couple were attacked by "at least four men", police said. Some reports said the attackers included the bus staff.
On Monday, senior police officer Chhaya Sharma appealed to the public to help identify the owner of the bus.
At a press conference, police showed camera recordings of a white luxury bus which they said could have been the one used in the crime.
Reports say the alleged rape took place on a "white line" bus - such services run by private operators have higher fares and fewer stops. They employ more upmarket vehicles to supplement the city's public bus network.
The incident has led to outrage in the city.
"This is a shocking incident. I hope that the guilty are punished for the heinous crime," Delhi's Women and Child Development Minister Kiran Walia said.
The chairperson of the National Commission for Women, Mamata Sharma: "Such incidents are increasing in Delhi. Police and government should be really alert.
"We will get an inquiry done and will also inquire with the police as to what action has been taken so far."