Inmates of three prisons, located in Colombo city, Kalutara town and in Vavuniya town, are demanding that they be charged for their crimes or be released, Xinhua reported.
The fast began last week with the participation of 80 inmates of the Colombo prison but the number rose to 234 over the weekend with prisoners from two other prisons joining the protest.
Most of the prisoners have been in jail for over three years without being charged, said Tamil National Alliance (TNA) legislator MA Sumanthiran, adding that a discussion will be held on Monday with prison authorities regarding the long-term detainees.
The European Union had earlier this month called for a fair trial for long-term detainees in Sri Lanka.
Jean Lambert, chair of the European Parliament Delegation for Relations with countries in South Asia, has said that the EU Parliament's position on long-term detainees in Sri Lanka is that they should be either charged or released.
Hundreds of suspected Tamil Tiger rebels and other political prisoners are in Sri Lankan prisons, some of whom have not faced trial or a charge for years or months since being arrested, according to minority Tamil political parties in Sri Lanka.
Most of the prisoners were arrested while emergency regulations were in place during the war between the Tamil rebels and the military.
The war ended in May 2009 with the defeat of the rebels. Most of the emergency regulations were also subsequently lifted.