Khatami, who served as president of Iran from 1997 to 2005, vowed to remain steadfast in his opposition against conservatives through his Reformist movement, Iran's state-funded broadcaster Press TV reports.
"We will continue to stay critical of the current power trend, of course within the framework of a movement that supports Islam, the Islamic republic and the (1979) Revolution," he told university students at an opposition rally.
His comments come as government officials warn against mass demonstrations as Iran braces for the anniversary Wednesday of the seizure of the U.S. Embassy in Tehran in 1979.
Khatami formed part of the so-called Green Movement in Iran that rallied around former Prime Minister Mir Hossein Mousavi.
Mousavi complained of widespread fraud in his defeat at the hands of Iranian President Mahmoud Ahmadinejad in June elections.
Iran in the wake of the June vote witnessed some of the most violent demonstrations since the Islamic Revolution.