In an apparent attempt to discourage protesters, an Iranian pro-government news agency has claimed that armed opposition groups plan to fire on people participating in a rally set for Sunday afternoon.
The report from the hardline Fars news agency said that teams from the Mujahedeen Khalq, an Iraq-based Iranian opposition group, have entered the country to shoot people during the protest.
Iran's opposition has called for a rally to mark a week since the deaths of two people in clashes between security forces and opposition protesters in Tehran. The opposition maintains the dead were killed by government forces.
Last week's rally called by opposition leaders Mir Hossein Mousavi and Mahdi Karroubi in solidarity with the Egyptian uprising was the largest demonstration by the opposition in more than a year.
On Sunday, Mr Karroubi sent an open letter to judiciary reaffirming his backing for opposition demands on his website Sahamnews.net.
"God willing, there will be no doubt in Mahdi Karroubi continuing to defend the rights of Iranian people and I will stand by the people until my final moment," the statement said.
The opposition leader said hundreds of hardliners have gathered in front of his home over the past night and filled the air with anti-opposition chants and threats to his life.
Both opposition leaders have been under tight house arrest since calling the 2009 re-election of President Mahmoud Ahmadinejad fraudulent.
Hundreds of thousand of people poured into the streets in protest to the result of the election. Opposition says scores were killed in massive crackdown on the protest demonstration. Government brought the number to about 30.
Authorities detained hundreds and sentenced about 80 of them to prison terms from six months to 15 years.