There's a new twist in a case that's triggered widespread protests across India.

The driver of a bus in which a young woman was gang-raped and fatally injured three months ago allegedly hanged himself here in New Delhi's Tihar Jail.

Ram Singh was one of the five men and one minor on trial for the attack on the 23-year-old in the Indian capital.

Police described him as the ringleader.

The man's lawyer questioned suicide as the cause of death on Monday saying "there were no such circumstances that could force him to commit suicide".

Officials are performing an autopsy to determine why and how the bus driver died, and prison authorities have ordered a magisterial inquiry.

Police allege the six suspects attacked the woman and her male companion as the couple rode the bus home after watching a movie on December 16.

The woman was raped and tortured with a metal bar - and both were also beaten before being thrown onto the road.

The woman died of