of the economy during the eight-year war with Iraq's Saddam Hussein in the 1980s. The conflict erupted at a time when the revolution that ousted the Shah was still in its infancy.
Mousavi has, however, been out of the public eye for nearly two decades -- his last high profile position being prime minister under the presidency of Ayatollah Ali Khamenei, the current supreme leader of Iran.
The post of premier was scrapped by an amendment to the constitution in 1989. Since then, the moderate Mousavi has been absent from the top echelons of national politics and had kept away from the media spotlight.
But in a televised debate with Ahmadinejad which saw bouts of mudslinging, Mousavi declared that he decided to return to national politics in order to stop the hardliner from 'endangering' Iran.
At one point during the debate, Mousavi strongly defended his intellectual wife when Ahmadinejad charged that she had received her doctorate without "attending university exams."
Mousavi hit back, saying his wife Rahna Zahnavard had worked for 10 years for her Phd. His firm defence of his wife's credentials should ensure him strong support from women voters.
From the day he registered his candidacy, Mousavi, despite lacking charisma, has steadily emerged as the main rival to Ahmadinejad on the back of a lively campaign.
Young men and women are rallying in many city squares and busy crossings, shouting "Mousavi we support you" and seeking to lure passers-by with campaign literature and pictures of their candidate.
Accusing Ahmadinejad of 'undermining the dignity' of Iran, Mousavi has promised more rights to women if elected.
His handling of the economy during the war with Saddam's forces also wins praise.
"He has a good record as a prime minister," said retired schoolteacher Shahbad from the northern city of Gorgan during one of Mousavi's provincial tours.
"He handled the economy very well during the Sacred Defence," he said referring to the war.
Mousavi's supporters believe he can fix the deteriorating economy and galloping inflation which is hovering around 24 percent.
Ahmadinejad has overseen a sharp increase in public spending which critics have derided as populist and inflationary.
Mousavi has also promised that if elected he will seek to change the "extremist" image that Iran has earned abroad during Ahmadinejad's presidency and has not ruled out the possibility of negotiating with US President Barack Obama.
In his bid for the presidency, Mousavi has the key support of reformist former president Mohammad Khatami who won sympathy in the West for his advocacy of a "dialogue of civilisations" during his eight years in office to 2005.
But Mousavi has also warned that if elected he will not go back on the tough position on Iran's nuclear programme taken by Ahmadinejad.
"I do not think any government will dare to take a step back in this regard, since people will question the decision. Given the long-term interest, we are obliged not to back down on this or other similar issues," he said in a press conference in April.
However, Ahmadinejad's supporters do not mince words when pointing out his political absence of two decades.
"We do not know what he did in these 20 years, while Ahmadinejad has done good for workers," said Hossein Khorsavi, 63, a retired commerce ministry employee from Islamshahr, a city of migrant workers on the outskirts of Tehran.
Mousavi is also a member of the Expediency Council, Iran's top legislative arbitration body and served as a presidential adviser from 1989 to 2005.
He is also head of Iran's Art Academy, set up to safeguard the Islamic republic's national heritage.
Mousavi was born on September 29, 1941 in the town of Khameneh, hometown of the ancestors of supreme leader Khamenei.
Although the two are unrelated, they were among the founders of Islamic Republic, the now defunct party set up during the early years of the 1979 revolution.