Sri Lanka's ex-army chief was arrested on 8 February after losing his presidential bid to incumbent President Mahinda Rajapaksa.
He played a key role in the defeat of the separatist Tamil Tigers after a bloody civil war that lasted three decades. Afterwards, he entered the political arena to wrest power from President Rajapaksa.
The government has also accused him of planning a coup and plotting to assassinate the President, charges that will not however figure at the court martial. A three-member panel of two-star generals is presiding over the trial.
If he is found guilty, General Fonseka could get up to five years in jail.
Hearings began this morning at 9.30 (local time) at Navy Headquarters in Colombo.
Sri Lanka's former Chief Justice, Sarath Silva, on Monday said the trial before a military court was unconstitutional because General Fonseka is not subject to military law. If he has to be tried, it should be in a civilian court.
Fonseka's daughter, Apsara, has accused the government of trying the retired general in order to remove him from the 8 April parliamentary election, in which he intends to stand.