Ram Singh, one of the main accused in the Delhi gang rape case, has allegedly committed suicide inside Tihar jail on the outskirts of the city.
Police said the suspect hanged himself despite being kept on suicide watch.
Singh was one of five men charged with the gang rape and murder of a 23-year-old paramedic. The incident, which allegedly took place on a private bus in Delhi in December, caused national and international repercussions.
"He should have faced the gallows and should have been hanged publicly," said the gang-rape victim's brother.
Singh's body has been taken to the Deen Dayal Upadhyay Hospital in New Delhi for a post mortem examination.
All five accused were due to appear in a special fast-track court for trial. A sixth suspect faces a separate trial before the Juvenile Justice Board.
Police officials reportedly said that the death would not affect the ongoing trial.
The federal government has ordered an inquiry into the death, said RPN Singh, home affairs minister.
"There was no sign he was depressed. Mentally he seemed fine. I am shocked to hear this news. There were security concerns, so we wanted the case to be transferred out of Delhi, but Ram Singh was not under stress," said Singh's lawyer, VK Anand, according to the news website FirstPost.
"He looked happy. He had his dinner last night. Why should he commit suicide?" another lawyer, Vivek Sharma, told the BBC.
"Only an inquiry can tell us what the circumstances of his death were," said lawyer and human rights activist Vrinda Grover.