Iran's opposition will proceed with a planned rally in Tehran despite supreme leader Ayatollah Ali Khamenei warning the groups against holding protests, an aide to defeated presidential candidate Mehdi Karroubi said.
"The demonstration plan has not been cancelled and accordingly it must be held this afternoon," Reuters reported the aide as saying.
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The BBC reported Mir Hossein Mousavi's wife, Zahra Rahnavard, wrote on her Facebook page that the rally would go ahead, while an announcement reportedly made on the Web site of Mousavi's Kalameh newspaper said he would make an "important" statement to the Iranian people.
Deputy national police commander Ahmadreza Radan said Iranian police will deal firmly with any illegal gatherings.
"I should emphasize that all protests held in the past week were illegal and beginning today any gathering critical of the election would be illegal," Reuters reported Radan as saying. "And police will deal with it (the protests) firmly and with determination."
Mousavi received a stern warning Saturday not to encourage his supporters to take to the streets, a day after Khamenei warned opposition leaders to end street protests or be held responsible for any "bloodshed and chaos" to come.
It was unclear if Mousavi would attend the rally or how large it would be.
Iran's Interior Ministry reiterated the warning to Mousavi on Saturday, saying he would "be held responsible for the consequences of any illegal gatherings." The ministry also accused the 67-year-old former prime minister of supporting protests that "have lead to the disruption of security and public order," State Security Council secretary, Abbas Mohtaj, said in a statement on the ministry's Web site.
The warnings place Mousavi at a pivotal moment. He can either back down or risk a crushing response from police and the forces at Khamenei's disposal — the powerful Revolutionary Guard and their volunteer citizen militia, the basij.
There also are questions about Mousavi's ability to control his own followers, many who are waiting for a clear response to Khamenei's edict on Friday before Saturday's planned rally.