"I will go out of my way to expose anyone who has committed war crimes," Fonseka told reporters. "I will not protect anyone, from the very top to the bottom."
Fonseka was arrested shortly after contesting presidential elections in January and faces court martial proceedings for allegedly dabbling in politics before he quit as army chief in November.
He spoke with reporters inside parliament after being escorted from custody to attend Thursday's session as an opposition MP. He won a seat at April parliamentary polls.
Fonseka said the government was afraid that he would expose anyone found guilty of rights abuses.
"It is not patriotic to protect anyone who has caused injustices to Tamil youth," Fonseka said, adding that he himself was not personally aware of any instances of abuses but wanted all allegations investigated.
Fonseka, the architect of the military offensive against the Tamil Tigers, has accused the president's brother, defence secretary Gotobhaya, of ordering the execution of surrendering rebels in the final stages of fighting."There were a lot of things said at the time (of the final offensive)," Fonseka said. "If there is any international investigation, I will fully support that."