He has gone from colorless insider to political rock star - a graying, bearded veteran of the Islamic regime who now stands at the forefront of a youth-driven movement fighting for change.
Despite his newfound fame, Mir Hossein Mousavi still works out of his old office at the Iranian Art Academy and lives in the same unassuming brick home in a middle-class district of Tehran as before, according to an aide.
When he appears in public, such as at a rally yesterday in Tehran, crowds surge around his car, chanting his name.
It's unclear what has propelled this calm, deliberate architect and artist - who twice refused to seek the presidency - into a confrontation with the ruling establishment of which he was once a part.
Nor is it clear how Mousavi will respond if the opposition movement transforms from a campaign against alleged fraud in last Friday's election into a challenge to the core value of the Islamic Republic - that senior clerics have the final say on big issues.
Even during the election campaign, Mousavi was less critical of President Mahmoud Ahmadinejad than another challenger, former parliament speaker Mahdi Karroubi, who received only a fraction of the vote.
For years, he remained out of the political limelight, painting pictures - mostly with religious themes - and designing buildings.
Nevertheless, a 67-year-old nearly devoid of personal charisma has become the champion of a generation inspired by the hope of change, organizing protests with technologies such as mobile phones and Internet that did not exist when their parents overthrew the U.S.-backed shah in 1979.
In his new role, Mousavi has displayed a common touch - something Ahmadinejad has also portrayed in an effort to identify with millions of poor Iranians.
Although a number of Mousavi's followers have been arrested, aides insist he has maintained his old routine, even as his challenge to the powerful clerical establishment is growing.
"Mousavi goes to his regular job as the head of Iran's Art Academy and lives with his family in the same place he lived before the election," said Qorban Behzadian Nejad, head of his campaign headquarters.
"At the same time he pursues his activities for nullification of the election."
Much of Mousavi's appeal among Iranians eager for change probably stems simply from the fact that he is not Ahmadinejad, a hard-liner who has failed to deliver on economic promises and who seems to relish provocative statements - from calling protesters "dust" to denying the Holocaust.
Hard-liner was once a term also applied to Mousavi during the early years of the Islamic republic.
Like many educated young Iranians, Mousavi was drawn into the Islamic movement by the writings of Ali Shariati, an Iranian sociologist who tried to harness Islamic and socialist ideals in the cause of social change until he died in Britain under mysterious circumstances in 1977.
Those ideas inspired idealistic young Iranians to join Ayatollah Ruhollah Khomeini in his drive to oust Shah Mohammed Reza Pahlavi in 1979.
Mousavi played an active role in the 1979 revolution, then served as foreign minister and later prime minister of the new Islamic republic from 1981 until 1989.
For most of those years, the nation was at war with Iraq, and the government displayed little tolerance for dissent. Critics faced arrest and execution as traitors.
During that period, Mousavi's office also approved a secret nuclear program, which has since put Iran on a collision course with the United States.
But he also won praise for his management of Iran's economy and for trying to end the country's international isolation - policies that often put him at odds with then-President Ali Khamenei, now the supreme leader and ultimate authority in the country.
After the post of prime minister was abolished, Mousavi left the public stage. He spent most of the next decade working as an architect and helping rear his three daughters.
Twice - in 1997 and 2005 - he refused pleas by reformists to run for president.
This year, in a reversal, Mousavi accepted the challenge. He promised economic reform, freedom of expression, and a campaign against economic corruption.
He also pledged to review laws discriminating against women, remove the ban on privately owned television stations, and curb the power of the supreme leader by taking control of security forces.