Cleric Akbar Hashemi Rafsanjani, who will go head-to-head with Mr Ahmadinejad in a second run off on June 24, joined failed reformist candidate Mostafa Moin and moderate cleric Mehdi Karoubi in challenging the tally.
The men have all made unprecedented public allegations that a dirty tricks, including a smear campaign and vote buying on a massive scale, was employed to ensure Mr Ahmadinejad’s shock success.
They claim Iran’s Revolutionary Guard and Basij militia are responsible for voting irregularities which they accused the Guardians Council of improperly handling.
Mr Ahmadinejad, the 49-year-old mayor of Tehran, hit back at his opponents saying they were sore losers and out of touch with ordinary Iranians.
In his bid to become Iran’s first non-cleric president since 1981, Mr Ahmadinejad has campaigned hard to win over the country’s poor hoping his own humble background will strike a chord with impoverished communities.
Mr Rafsanjani has taken to the universities to drum up support from students.
In a tense and at times volatile appearance at Tehran University, Mr Rafsanjani urged leftist students to set aside grievances about the lack of real power exercised by elected officials and vote.
“I believe that I was the prime mover in establishing reforms, and (outgoing President Mohammad) Khatami’s government took further steps. Definitely it should go on,” Mr Rafsanjani said, pledging to stay true to the reformist agenda he set as president from 1989 to 1997.
He also promised to stand firm if the Guardians Council interfered with the outcome of this week’s poll.
“If the Guardian intervenes to change the vote, I will act to stop them.”
But Mr Rafasanji, a 70-year-old former conservative who has been at the centre of Iranian politics since the 1979 Islamic Revolution, faced vocal protests from some students over the imprisonment of a journalist.
Akbar Ganji is Iran’s longest serving gaoled-reporter, behind bars for five years for drawing a connection between Mr Rafsanjani and the serial killings of dissidents.
As the battle between left and right heats up, Iran’s main reformist party, the Islamic Iran Participation Front, has called on all supporters to pull together and return the cleric to the presidency to “avert a catastrophe”.