Some 27 prisoners, mostly belonging to the proscribed Al-Umma, refused to take food in the morning and afternoon and appealed to the government to release him, jail authorities said.
Abu Thahir has suffered multiple organ failure, followinga congenital disease and is undergoing treatment in a private hospital here amid security.
He is undergoing imprisonment in the 1998 serial blasts case and for murder of an assistant jailor of Madurai Central Prison, they said.
His family has sought his release on humanitarian groundsand the fasting prisoners have sent a petition to State Chief Minister O Paneerselvam, seeking his release, jail officials said.