NEW DELHI (REUTERS) - India's Supreme Court on Thursday rejected a petition by the head of the unlisted Sahara conglomerate to be released from custody and said the matter would next be heard on March 25.

Sahara Chairman Subrata Roy, 65, was arrested on Feb 28 after failing to appear at a contempt hearing in the long-running legal battle between the unlisted group and the Indian securities regulator over repayment of billions of dollars to investors in outlawed bonds.

Roy has been held in a Delhi jail since March 4. He has not been charged with a crime and was not present in court on Thursday.

Last week, the court rejected a Sahara proposal to give bank guarantees of 225 billion rupees ($4.7 billion) within three to six months, as well as a subsequent proposal to pay a total of 174 billion rupees in six instalments through July 2015.

The court has asked Sahara to come up with a "concrete and acceptable" plan to refund investors.