A former Iranian nuclear negotiator running for president used his first television appearance of the campaign to reject accusations he had been too soft in talks with world powers.
The most prominent moderate candidate in an election dominated by hardliners, cleric Hassan Rohani, nuclear negotiator from 2003 to 2005, oversaw an agreement to suspend Iran's fledgling uranium enrichment-related activities.
Iran has since stepped up its nuclear program which many countries, particularly in the West, fear is aimed at acquiring a weapons capability, something Tehran strongly denies.
Hardliners see the nuclear program as a matter of national pride and any concession to outside pressure an affront to Iran's sovereign rights. The current nuclear negotiator, Saeed Jalili, is campaigning for president on his record of giving no ground in talks.
Western powers are watching the June 14 election to see whether President Mahmoud Ahmadinejad's successor will set a new tone in talks - several rounds of which in the last year have failed to defuse tensions over the nuclear program which Israel has said it could use military force to stop.
In a spirited exchange on state television on Monday, Rohani said allegations he had halted nuclear development were "a lie" and suggested his interviewer was "illiterate".
"It's good if you study history," a smiling Rohani, dressed in the traditional clerical garb, told the suited interviewer. "We suspended it? We mastered the (nuclear) technology!"
The 64-year-old argued the Islamic Republic had expanded uranium enrichment during his tenure while demonstrating the programme's peaceful nature and preventing a U.S. attack.
"We didn't allow Iran to be attacked," he said, referring to the U.S. military campaigns in Afghanistan and Iraq.
"They (the U.S.) imagined tomorrow or the day after, it would be Iran's turn."
Nuclear policy is ultimately decided by Supreme Leader Ayatollah Ali Khamenei, and all candidates stress Iran's right to peaceful nuclear energy and deny plans to build nuclear weapons.
Analysts say voters are more likely to decide on candidates based on how they would reinvigorate an economy suffering from high unemployment and inflation.
But the nuclear issue has been "used to discredit rivals" in the early days of the campaign, said Dina Esfandiary, an Iran analyst at the International Institute for Strategic Studies.
Jalili's camp is trading on its hardline attitude to the nuclear programme. In the last five years, Jalili, seen as rigidly devoted to Iran's Islamic revolutionary ideals, has overseen a hardening stance in talks with world powers.
"Our national interests and security were tarnished and hurt," said Ali Bagheri, Iran's deputy nuclear negotiator who is supporting Jalili's campaign, in a recent speech, referring to Rohani's tenure under reformist President Mohammad Khatami.
"The fate of that period was unhappy and God forbid it should be a period that we return to."
Several rounds of nuclear talks between Iran and six world powers - the United States, Russia, China, France, Britain and Germany - have failed to reach an agreement.