Mir Hossein Mousavi was the last Iranian prime minister
The influential former Iranian Prime Minister, Mir Hossein Mousavi, will contest the Iranian presidential election on 12 June.
Mr Mousavi, who speaks Persian, English and Arabic, held office during the Iran-Iraq War of 1980-1988.
His candidacy may split voters opposed to Mahmoud Ahmadinejad, the current president who is likely to stand again.
His rivals include ex-President Mohammad Khatami and Mahdi Karroubi, both of whom are leading moderates.
"We have to avoid wasting resources for short-term interests and unworthy political aims," said Mr Mousavi in a statement - in an apparent criticism of President Ahmadinejad.
He also praised Iran's technological achievements, especially in the nuclear field - a source of tension between the fourth-largest producer of oil and the United States.
Mr Mousavi was the prime minister under the presidency of Ayatollah Ali Khamenei, who is now the Islamic Republic's supreme leader.
He is also a member of Iran's Expediency Council which is the country's top political arbitration body.