In letters and phone calls to BBC Tamil, ex-militants say they have been "tortured and beaten" in the centres.
They accuse camp guards of being corrupt and demanding bribes before releasing detainees.
The government says all those being held in custody following the end of the war are being well cared for.
It has consistently refused to allow any kind of independent investigation into allegations of human rights abuses in the final weeks leading up to the end of the war in May 2009. More...