"Having nuclear technology for peaceful purposes without being a threat to the world is our strategic objective," Mousavi said in a speech to his election campaign managers on Tuesday, a copy of which was obtained by AFP on Sunday.
"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."
Mousavi is contesting the June 12 presidential election that will pitch him against incumbent hardline President Mahmoud Ahmadinejad, who is yet to formally announce his candidacy.
Despite being the world's number four crude oil producer and holding the second largest gas reserves, Iran insists it needs nuclear power to sustain a growing population, saying its fossil fuel will run out in the coming decades.
But the West, spearheaded by Tehran's arch foe Washington, fears the Islamic republic's nuclear drive is aimed at making the atomic bomb.
Iran has already been slapped with three sets of UN sanctions for defying international demands to freeze uranium enrichment, the process which makes nuclear fuel but also the core of a nuclear bomb.
Mousavi was prime minister between 1981 and 1989, when the post was scrapped.
He is a member of the Expediency Council, the top political arbitration body, and heads Iran's Art Academy, which was established to safeguard the national heritage.
He also served as presidential advisor from 1989 to 2005.
Former parliament speaker Mehdi Karroubi, a reformist, is also planning to stand in the June election.
