Wednesday marks the 30th anniversary of the takeover of the U.S. embassy in Tehran.
"We are announcing that only anti-American rallies in front of the former American embassy in Tehran are legal. Other gatherings or rallies on Wednesday are illegal and will be strongly confronted by the police," Tehran police said in a statement, IRNA reported.
Anti-U.S. rallies in the capital will take place outside the former embassy, now called the "den of espionage" in Iran, to mark seizure of the embassy after Iran's 1979 Islamic revolution when radical students took 52 Americans hostage for 444 days.
Reformist websites have called on people to gather outside the Russian embassy instead, in apparent protest at Moscow's recognition of President Mahmoud Ahmadinejad's re-election on June 12, which sparked Iran's worst unrest for three decades.
The opposition says the vote was rigged to secure Ahmadinejad's win. The authorities deny the charge.
Defeated moderate presidential candidates Mirhossein Mousavi and Mehdi Karoubi have urged their supporters to take to the streets on Nov. 4.
A reformist website said Karoubi will attend the rally outside the former U.S. embassy.
(Editing by Louise Ireland)
