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 week of unrest in Iran.
State television aired interviews on Saturday with critics of the protests sparked by a disputed June 12 presidential election, urging Iranians to unite behind the government and suggesting only the West gained from Iran's troubles.
Helicopters criss-crossed Tehran 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 were deployed in force, firing teargas and 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.
Government restrictions prevent correspondents working for foreign media attending demonstrations to report, and the scale of any injuries or arrests was unclear.
State television said a "number" of people were injured.
Mousavi, focus of the biggest protests since the Islamic Revolution ousted the U.S.-backed Shah in 1979, said 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 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.
The authorities reject charges of election fraud.
State television said rioters smashed windows of banks and burned buses. They also aired interviews with people critical of the demonstrations that have racked major oil and gas producer Iran since the announcement of the election results on June 13.
"We all should listen to our leader (Khamenei) and preserve calm," said one unnamed woman, aged around 40. "Otherwise we will make our enemies (the West) happy."
State television said Iran had arrested members of the exiled, opposition Mujahideen Khalq Organisation (MKO) it accused of "terrorist activities" including setting buses on fire and destroying public property.
Iranian newspapers carried a letter to Mousavi from Iran's police chief Ismail Ahmadi Moghaddam, in which he warned that officers would "decisively confront" any further unrest.
Mousavi is himself a product of the Islamic establishment that has dominated Iran since 1979 and opposing that establishment may sit uneasily on his shoulders. But the demonstrations of the last week, swelling to hundreds of thousands, appear to have acquired a powerful momentum.
Beyond the violent confrontations with police, it was a day fraught with symbolism for the Islamic Republic.
A suicide bomber blew himself up at the mausoleum of Ayatollah Ruhollah Khomeini, police and state media said - an attack likely to stir passions in a country where the father of the Islamic revolution is deeply revered. The identity of the bomber was not known.
Another reminder of 1979 came as darkness fell, when supporters of Mousavi sent cries of Allahu Akbar (God is greatest) echoing across the rooftops.
United States President Barack Obama, in the forefront of diplomatic efforts to halt an Iranian nuclear programme the West fears could yield atomic weapons, urged Tehran to "stop all violent and unjust actions against its own people".
"The Iranian government must understand that the world is watching. We mourn each and every innocent life that is lost," Obama said in a statement.
Iran's highest legislative body said it was ready to recount a random 10 percent of the votes cast in the election to meet the complaints of Mousavi and two other candidates.