Reformist leaders in Iran have urged supporters to rally behind former President Akbar Hashemi Rafsanjani in Friday's presidential run-off.
Akbar Hashemi Rafsanjani is a pragmatic conservative
They want to prevent a victory for the hardline conservative candidate and mayor of Tehran, Mahmoud Ahmadinejad.
Few had predicted Mr Ahmadinejad would be the one facing Mr Rafsanjani.
The former president has urged Iranians to support him against "extremists" who had "tarnished" the first round of the election, the AFP news agency reports.
At the same time, traditionalist conservatives have been urged to put their full weight behind Mr Ahmadinejad - a former revolutionary guard who beat five other candidates and nearly topped Mr Rafsanjani's vote in last week's poll.
No Iranian presidential election has gone to a second round before.
Mr Ahmadinejad's shock election success has frightened Iran's reformists and even moderate conservatives, says the BBC's Sadeq Saba in Tehran.
Many people in Iran are now concerned that a possible final victory for the hardline mayor of Tehran may erode some of their social freedoms gained in recent years, our correspondent says.
Akbar Hashemi Rafsanjani - 21%
Mahmoud Ahmadinejad - 19.5%
Mehdi Karroubi - 17.3%
Source: Iranian interior ministry
A spokesman for the defeated reformist candidate Mostafa Moin has told the BBC that the mayor of Tehran represents what he called "a dangerous military and anti-democratic force in the country".
He said all liberal elements in Iran should now form an alliance to stop the hardline politician coming to power.
Several candidates who failed to get through to the second round have made allegations of vote-rigging and fraud.
They say Mr Ahmadinejad's supporters ordered Islamic paramilitary forces to vote for him.
Two of the most significant reformist organisations - the Islamic Revolution Mujahideen Organisation and the Islamic Iran Participation Front - issued statements saying they opposed fascism and the involvement of the army in politics.