The security forces were deployed on Vali Asr street, which bisects the city from north to south, and in nearby areas, witnesses said.
Reformist opposition supporters called on their websites and Facebook for a silent march on the pavements of the street to mark the 2009 presidential election, which saw Ahmadinejad elected for a second term by a landslide.
Ahmadinejad, who is now challenged by the ruling conservatives in his own camp, defeated opposition leaders Mir Hossein Mousavi and Mehdi Karroubi in a poll they continue to reject as fraudulent.
Their opposition to Ahmadinejad sparked a wave of unrest in the Islamic republic, plunging the regime into one of its worst political crises.
The protests left scores dead while thousands, including many close to the opposition as well as journalists, were arrested, with some handed long prison sentences.
Mousavi, a former prime minister, and Karroubi, a former reformist parliament speaker, have been under house arrest for the past four months.
Their access to the outside world was cut off after they called for a demonstration in February, gathering thousands of people.