"The police will act against any illegal gathering on the 13th of Aban," deputy police chief Ahmad Reza Radan told the Mehr news agency, referring to the November 4 anniversary.
Several reformist websites are urging opposition supporters to gather on Wednesday and protest "peacefully" and not to resort to "violence even if they are attacked."
Main opposition leader Mir Hossein Mousavi, in a statement on his website Kaleme.com, has also hinted at a possible protest rally on Wednesday.
"The 13th of Aban is a… rendezvous so we would remember anew that among us it is the people who are the leaders," Mousavi said.
Radan said the police has the "obligation to prevent any disturbance of order in society."
November 4 has emerged as an anti-American day in Iran, with thousands of Iranians, mostly students, gathering annually outside the US embassy building to shout slogans against the United States and Israel.
The event marks the capture of the embassy on November 4, 1979 — just months after the Islamic revolution toppled the US-backed shah — by radical Islamist students who took American diplomats hostage for 444 days.
This year the event could be marked by street protests against Ahmadinejad, whose re-election on June 12 triggered the worst political crisis in the 30-year history of the Islamic republic.
On September 18, opposition supporters turned an annual pro-Palestinian rally into a similar anti-Ahmadinejad protest.
In the unrest that followed the June election, security forces arrested some 4,000 protesters, reformists and journalists, and dozens of people were killed in the clashes.
Most of those detained have since been released, but around 140 are being prosecuted, among them some senior reformist politicians, in what opposition leaders have labelled "show trials."
