Tehran general prosecutor Abbas Jafari Dolatabadi was speaking after opposition leader Mirhossein Mousavi said on his website on Sunday that the legitimacy of clerical rule was waning due to its "repressive measures."
"Even though some go on trying to agitate the atmosphere in society with statements… they've been given the answer by the people," Dolatabadi said in comments on state news agency IRNA.
"We will not witness street demonstrations and we will not allow anyone to come to the streets to disrupt public security without proper permits."
The disputed June re-election of President Mahmoud Ahmadinejad plunged the Islamic Republic into its most serious internal crisis in its 30-year history and created a rift within the ruling establishment.
Hardliners have accused reformist opposition leaders of inciting unrest and called them enemies of God, a crime punishable by death under Iran's Islamic law.
"Even though threats against the revolution will not come to an end, we will not succumb and certainly one day in the not so distant future despair will take them and they will surrender," Dolatabadi said.
"The file on the election has been closed and law enforcement agencies have been asked to preserve security," he said, referring to often rowdy large street celebrations involving fires and fireworks on the last Tuesday of the Iranian year, March 16.
The Iranian new year begins on March 21.
Opposition supporters have defied government warnings against staging illegal rallies and have sought to hijack official demonstrations to hold anti-government protests.
The domestic strife comes as Iran faces growing Western calls for more sanctions after Ahmadinejad ordered production of higher-grade uranium, stirring fears that Tehran aims to make nuclear bombs, not just fuel for civilian use.
(Reporting by Hashem Kalantari; Writing by Andrew Hammond; Editing by Jon Hemming)
