President Mahinda Rajapakse has promised "due process" will be followed in dealing with Fonseka, the president's website said.
Fonseka was arrested Monday by military police after allegations he was involved in a conspiracy against the government.
"The president has assured opposition leader Ranil Wickremesinghe that there was no reason for any concern over General (retd.
) Sarath Fonseka who is in detention," the president's website said.
It quoted the president as saying that "no one was above the law" and said Fonseka's candidacy in the January 26 presidential polls had nothing to do with his arrest.
Sri Lanka's Supreme Court said Friday it would hear a petition submitted by Fonseka's wife calling for his arrest to be ruled illegal and ordered the state to allow the former army chief family visits and medicines.
The court was scheduled to reconvene February 23 for a further hearing.
Fonseka was arrested two weeks after being defeated in the elections by a solid