"The main policy of my administration will be an oil-free economy. It means the budget dependency on oil will be lowered to zero," said pro-reform candidate Mohammad Reza Aref in the discussion aired on state television.
His stance was echoed by the other candidates, all of whom were chosen out of nearly 700 would-be contenders by hardline electoral watchdog the Guardians Council.
The eight engaged in a question and answer session during the debate on the economy, but refrained from making direct attacks on their rivals or criticising them.
The economy has been hit hard by US and EU sanctions targeting its petrodollars over Tehran's disputed nuclear programme of uranium enrichment.
World powers suspect a military motive for Iran's nuclear project, a charge Tehran denies.
In today's first debate, top nuclear negotiator-turned presidential candidate Saeed Jalili said the sanctions "present an opportunity if we lower our dependency on oil".
Jalili, a conservative frontrunner seen close to Khamenei, said that under his presidency Iran would move to "avoid the sale of crude" and instead focus on increasing production of refined petroleum products.
Iran earned more than USD 100 billion from oil sales in 2011, but has seen that income halved since early 2012 because of the US and EU sanctions which have also made it more difficult for the country to repatriate its petrodollars.
Iranian officials refrain from releasing detailed data on oil exports.
According to the oil cartel OPEC, Iran's oil production dropped to 2.70 million barrels per day (bpd) in April, its lowest level in more than two decades. It produced 3.63 million bpd in 2011.
Inflation is officially running at more than 30 percent and foreign investments are drying up.
