The political grouping, the United Front for a Democratic Fiji, has called on the regime leaders to explain why a former military officer who took part in the 2006 coup was put on trial.
Pita Driti was last week found guilty of inciting a mutiny in 2010 and is to be sentenced this month.
The Front, which is made of all parties ousted from parliament in the coup, wants to know why Driti was excluded from the provisions of the new constitution that grant immunity to all military officers.
It says Driti was also part of the group that overthrew the elected government in 2006.
In a statement, the Front has also asked how Driti's trial could go ahead when some of the key people alleged to have been involced in the plot to oust the prime minister, Commodore Frank Bainimarama, and kill the attorney general, Aiyaz Sayed-Khaiyum, didn't make a court appearance.