Presidential hopeful Mir-Hossein Mousavi promises to "reveal" how missing oil revenues were spent during the four-year presidential term of Mahmoud Ahmadinejad, Press TV reported.
In a Monday address to supporters in downtown Tehran, Mousavi touched on a controversial National Audit report, which claims that more than one billion dollars in surplus oil revenues have gone missing from the country's currency reserve fund.
Mousavi asserted that "he will get to the bottom of the missing money and reveal how the government funds and oil revenues were spent in the past four years".
"The people have a right to know how the 1 billion dollars that was not returned to the treasury was spent. They have a right to know who has benefited from the rise and fall in tariffs," Mousavi said.
President Mahmoud Ahmadinejad and his administration have repeatedly rejected the findings in the audit report, describing the discrepancy as a miscalculation.
Also on Monday, Mousavi claimed that corruption levels had steadily increased in Iran under President Ahmadinejad, which he also accused of favoritism by employing his relatives in top government posts.
The four election rivals in Iran are engaged in a face-off that has had a strong focus on the economy and corruption.
Mousavi, who was Iran's last prime minister in the 80s, defended his administration's handling of the economy during his address and reminded his supporters that he served the country when the country was at war.
President Ahmadinejad also defended his economic policies and said that an "accounting error" has led to the report that $1 billion has gone missing from the country's currency reserve fund.
In an interview on Friday, President Ahmadinejad said the issue has its roots in a mix-up of currencies.
"In reality, they had added rials to dollars, and I declared on TV that this was a mistake," he said. "After numerous sessions, it became clear that it was an accounting error."