The prisoners had been arrested under anti-terror laws either during or soon after the war between the LTTE and the military.
Officials said that some 99 prisoners arrested on suspicion of having links with the rebels had launched the fast.
The prisoners are demanding their release or to be formally charged.
The current Government, which took office last year, had assured last December that the prisoners will be released but only a few had been set free.
Tamil political parties staged protests in the North demanding the release of the prisoners in order to ensure the post war reconciliation process goes forward.
A similar protest was also staged in the capital Colombo near the main prison with the participation of some majority Sinhalese monks.
The former Government used anti-terror laws to arrest LTTE suspects and their supporters but the current Government has said it will replace the laws while maintaining National security. (Colombo Gazette)