Mr Rafsanjani has asked his countrymen to vote against “extremism” when he faces hardline challenger Mahmoud Ahmadinejad in Friday’s presidential run-off.
A win by Mr Ahmadinejad would leave every institution in Iran in the hands of the conservative, anti-Western far-right.
Mr Ahmadinejad came a surprising second in Friday’s first round vote, leaving reformists trailing in his wake and raising fears of a social and political crackdown if he wins power.
Breaking a silence he has conspicuously maintained since the vote, Mr Rafsanjani called on Iranians to support him in a vote against the extremists who he said had “tarnished” the poll.
A statement released to the press alleged smear campaigns and election day irregularities were behind Mr Ahmadinejad’s surprise showing.
At the same time centrist cleric Mehdi Karoubi, who came third in the poll, asked Iran’s supreme leader Ali Khamenei to prevent “illegal intervention” by the Revolutionary Guards and unelected Guardians Council in the elections.
Meanwhile, Iran’s vanquished opposition has called on its supporters to back Mr Rafsanjani even as prominent dissident figures maintained their calls for a boycott.
“The danger which threatens the country today is that of the barracks and soldiers directly intervening in the election and the politics of the country,” warned Iran’s main reform party, the Islamic Iran Participation Front (IIPF).
“We hold the hand of all partisans of freedom, democracy and human rights,” the party added, swallowing its pride and urging supporters to vote for Mr Rafsanjani.
There has already been a flurry of allegations by the opposition over vote-rigging in the election, which was also criticised by Iran’s arch-enemy the United States.
It was “hard to see how this election could certainly contribute to the sense of legitimacy of the Iranian government”, said US Secretary of State Condoleezza Rice.
The fifth place of main reformist candidate Mostafa Moin in the vote marked the latest blow for the movement which has seen its tenuous place in power slip away.
It also left them with the awkward choice of giving their explicit support to Mr Rafsanjani or urging a boycott of the vote and thus risk a victory by Mr Ahmadinejad.
Analysts believe it will be crucial for Mr Rafsanjani to mobilise the dispirited reformist camp since his opponent will be able to count on a core of highly motivated supporters.
Mr Ahmadinejad, a former Revolutionary Guard who has clamped down on Tehran’s cultural scene as mayor, is an even less palatable prospect to the pro-reform camp than Rafsanjani.
Many reformists fear that a win for Mr Ahmadinejad would roll back the cautious social reform that has taken place in recent years and further antagonise Iran’s relations with the international community, already tense over its nuclear programme.