Walid al-Bunni, a 46-year physician, was jailed for 2-1/2 years in October along with 11 other dissidents after holding a large meeting to revive opposition to 46 years of Baath Party rule in Syria.
According the indictment, al-Bunni is charged with making "false statements that weaken national morale".
It said he told a fellow inmate, in jail for car theft, Iran was exerting influence on Syria and Syria had contributed to a political crisis in Lebanon.
"I did not say any of this, and even if did, would I say it to such a swindler?" al-Bunni, wearing a striped blue prison uniform, told the court.