Iran has been rocked by street unrest since its disputed presidential election last June. Internet messages have circulated about new protests on Feb. 11, when Iran marks the 31st anniversary of the 1979 Islamic revolution.
Supporters of the pro-reform opposition have used such official occasions to stage new rallies in recent months, despite many arrests in a continuing crackdown by authorities.
Opposition leaders Mirhossein Mousavi and Mehdi Karoubi have called on supporters to attend next week's rally. An opposition website, Jaras, on Saturday said a youth group backing Mousavi also urged people to take part.
Government officials have rejected opposition charges that the June vote was rigged to secure the re-election of President Mahmoud Ahmadinejad. They portrayed the election protests as a Western-backed bid to undermine the Islamic establishment.
"Now that the different dimensions of the sedition are clear, we won't show any more tolerance," police chief Esmail Ahmadi-Moghaddam said, the ILNA news agency reported.
"Police will act firmly to defend the society's security and those who break the law will be dealt with severely," he said.
He said hundreds of people were arrested in connection with protests that erupted on Ashura — a ritual Shi'ite day of mourning that fell on Dec. 27 — with the help of tip-offs from the public after police published photographs of them. He said more such photographs of demonstrators would be issued soon.
Eight people were killed in clashes between security forces and opposition supporters on that day, in the most serious violence since the aftermath of the June 12 disputed election.
Moghaddam also reiterated a warning against the use of emails and phone text messages to spread the word of new protests, making clear police were monitoring such means of communication.
"The new technologies allow us to identify conspirators and those who are violating the law, without having to control all people individually," he said.
Last year's disputed election plunged Iran into its deepest domestic crisis since the Islamic revolution and exposed widening establishment divisions. Thousands of people were detained, including senior reformist figures, and dozens of people were killed in the unrest. (Writing by Fredrik Dahl; Editing by Charles Dick)
