Opposition leader Mirhossein Mousavi said the Islamic Republic must be purged of what he called lies and dishonesty, sending out a direct challenge to conservative rulers after a day of unrest across Tehran, Reuters reported.
State television aired interviews with critics of the protests, urging Iranians to unite behind the government and suggesting only the West gained from Iran's troubles.
Helicopters criss-crossed the city and ambulance sirens wailed into the night after streets emptied of protesters who had defied Friday's stern warning from Supreme Leader Ayatollah Ali Khamenei against further demonstrations.
Riot police had deployed in force, firing teargas, using batons and water cannon to disperse groups of several hundred Iranians who had gathered across the city. There were fears of further violence on Sunday in the country, a major oil and gas producer.
Government restrictions prevent correspondents working for foreign media attending demonstrations to report, and the scale of any injuries or arrests was unclear.
Mousavi, focus of the biggest protests since the Islamic Revolution ousted the U.S.-backed Shah in 1979, said June 12 elections that delivered an overwhelming victory to hardline anti-Western President Mahmoud Ahmadinejad were fraudulent and must be annulled. He said the fraud was months in the planning.
Mousavi, who claims victory in the poll, told supporters he was "ready for martyrdom", according to an ally. But he said he did not seek confrontation with the authorities.
"We are not against the Islamic system and its laws but against lies and deviations and just want to reform it," he said in a statement posted on his website at the end of a tumultuous day.
He said if authorities refused to allow peaceful protests they would face the "consequences" -- an apparent rejoinder to Khamenei's warning that opposition leaders would be held responsible for any bloodshed resulting from protests.
"The people expect from their officials honesty and decency as many of our problems are because of lies...The Islamic revolution should be the way it was and the way it should be," Mousavi said.