Opposition leaders Mirhossein Mousavi and Mehdi Karoubi have called on supporters to take part in peaceful protests to mark the first anniversary of a presidential poll they say was rigged, a reformist website reported in April.
Any anti-government rallies could raise the prospect of new clashes with the security forces, which quelled mass protests that erupted after the vote won by hardline President Mahmoud Ahmadinejad.
Hossein Sajedi, the capital's police chief, said "some media" had called for rallies on June 12, the ILNA news agency reported.
"Police will confront any illegal gatherings ... police are vigilant and in charge of public order and security," he said.
The authorities issued similar warnings ahead of planned opposition rallies in the months after the election, signaling their determination to crack down on any street unrest.
In their April statement, Mousavi and Karoubi urged groups and political parties to send requests to the Interior Ministry asking for permission to stage marches on June 12. The authorities have refused such requests in the past.
Despite the post-election crackdown, Mousavi and his allies have vowed to continue their reform movement, saying the Islamic state is in crisis and calling for detained opposition supporters to be freed.
The last major protest took place in December, when eight people were killed in clashes between security forces and opposition backers. Smaller protests occurred on Feb 11, when Iran marked the anniversary of the 1979 Islamic revolution.
The aftermath of the presidential vote, which plunged the Islamic Republic into its worst internal crisis in the past three decades, exposed deepening divisions in the political establishment of the major oil producer.
Thousands of people were arrested during the post-election protests, in which dozens of people were killed.
More than 100 people, including senior reformist figures, have received jail terms of up to 16 years. Iran has hanged two people sentenced to death in post-vote trials.
The West and human rights groups have condemned the executions and also the government's handling of the unrest.
