Opposition leaders say Fonseka was arrested on Monday as he was talking with them about the possibility of running in upcoming legislative elections.
President Rajapakse dissolved parliament on Tuesday and called snap polls for April 8. Parliamentary opposition leader and former premier Ranil Wickremesinghe met Rajapakse to demand Fonseka's release, as the Supreme Court took up a petition by the ex-army chief's wife challenging his arrest.
"The opposition leader also demanded to meet with General Fonseka who is detained at the navy headquarters in Colombo," an opposition statement said.
It said Rajapakse promised to take a decision on the fate of Fonseka, a retired four-star general, after military investigations into an alleged conspiracy against the government.
The government has yet to specify the charges Fonseka will face, but Defence Secretary Gotabhaya Rajapakse said he had clearly been plotting a military coup.
There was no immediate comment from the government about the m