"Be certain that there is no going back in this movement," he said.
Khatami joined opposition leaders in their bid to unseat Iranian President Mahmoud Ahmadinejad in the June presidential elections. Protests that followed the disputed victory for Ahmadinejad were some of the most violent in Iran since the 1979 Islamic Revolution.
Khatami warned his supporters that members of the conservative regime in Tehran were trying to sideline the opposition movement, reports Radio Zamaneh, an independent Persian broadcaster in the Netherlands.
"It is a pity that those who believe in the Islamic republic find it hard to speak openly," he said.
Conservative authorities accused the opposition, led in part by former Iranian Prime Minister Mir Hossein Mousavi, of attempting a velvet revolution, sending security forces into the streets to quell the unrest that followed the election.
Khatami maintained the opposition movement was proceeding within the confines of the Iranian legal system, stressing it was "not possible to protect the system for a long period through the use of the armed forces."