Ayatollah Ahmad Jannati, delivering the weekly Muslim prayer sermon in Tehran, also had an unusual warning for the security forces, telling them any soft treatment of those activists already in detention would be considered treason. "Nobody gives a flower to his murderer," he said.
Iranian authorities executed a fierce crackdown on the hundreds of thousands of protesters who poured into the streets in response to allegations that President Mahmoud Ahmadinejad won re-election in June through vote fraud.
Opposition groups say at least 72 protesters were killed in the unrest, while government officials insist only 36 people died. Hundreds were detained in sweeps and there were accusations that people were abused and even raped in custody.
Iran has also brought more than 100 people, including some of the most senior figures in the country's pro-reform movement, to trial on charges of plotting to overthrow Iran's clerical leadership. Three of the defendants have been sentenced to death.
Under the government's campaign, street protests fizzled and the opposition has been unable to decisively regroup.
The last significant protest was on Sept. 18, when tens of thousands of protesters — many chanting "Death to the dictator!" — rallied in defiance of a ban on the march by supreme leader Ayatollah Ali Khamenei and clashed with police.
That rally coincided with a government-organized anti-Israel protest held annually.
Opposition Web sites say another attempt will be made on Nov. 4, when there are calls for a rally coinciding with another yearly state-sponsored event — a day of marches to mark the anniversary of the 1979 student takeover of the U.S. Embassy.
Jannati, whose sermon was broadcast live on state radio, accused the opposition of seeking to transform the event into an outpouring of support for the U.S. Iranian authorities have sought since the unrest first broke out to portray the activists as tools of the West, particularly America and Britain.
"They want to show their pro-American and pro-Israeli nature on the day," Jannati said. "If they are allowed, they will say, 'Long live the U.S. and Israel."'
Jannati, a zealous supporter of President Ahmadinejad, heads a clerical body that oversees elections and parliamentary decisions.
Jannati asked intelligence services and the judiciary to deal harshly with detainees and said any officials who do not would be committing treason.